user106437: If an online store accepts crypto payments directly into its wallet there is a problem:
"Anyone can make a small transfer to any BTC address derived from the MPK so they can watch the store's and its client's funds as soon as the owner transfers funds from her wallet. I suppose possibly that the person who did this wanted to know if cracking the store worth the effort."
In a previous question a user replied that a solution to this is to use a crypto payment processor like BitPay but:

How they handle wallet privacy? Isn't it the same? The BTC only gets
to make a higher number of transactions between different addresses
but still, they can be traced to the store owner's wallet and her
client's afaik.
Or if the transaction is too small to match the price of the
purchase, BitPay returns the transaction to the originating address
and the BitPay address that accepted the transaction gets dumped so
it can't be used to trace BitPay's customer real address?
Another way I think this could be avoided as well is if BitPay
converts the payment to fiat, then to BTC again and then transfer the
funds to the customer wallet.
Wouldn't it be easier just to avoid payment processors and convert
the BTC to a privacy coin like Monero? For example using a
transitional wallet in the store to accept payments, making smaller
transactions to an exchange, converting to Monero and back to BTC and
then transfer to the owner's real wallet?
Could this method conceal the store owner and her client's wallet
balance to any potential hacker who makes a small transaction to any
store wallet MPK derived address?
Or the only way is to use a payment processor? If so, then how they
conceal their customer's wallet balance?

Thanks for your attention, it's a confusing subject for me and I hope to have written my questions as succinctly & understandably as possible.

user123812: TL;DR: your question is a valid one; there is no easy solution but CoinJoin and XMR swaps will help.
The 3rd party payment processors use wallet that is shared for all customers so this specific attack against specific merchant is not effective. But then you have another problem: the payment processor itself will leak your data, 1) ~legally to multiple partners, surveillance companies, regulators and LE 2) accidentally due to security breach, ransomware, etc 3) illegally, the data will simply be sold at some point. So it's a big no-no.
So what are the options?
To start with, use open source, self-hosted payment processors running against your own node. Examples include https://btcpayserver.org/ or https://cypherpunkpay.org/ (disclousure: author).
Then either:

pass coins through JoinMarket or Whirlpool
swap coins to XMR via near-instant, no-KYC exchanges (then wait because you cannot simply swap the same amount back to BTC)

The important thing is to work with individual coins (UTXO-s) or very small groupings of these - just to fit minimal thresholds required by the privacy tool of your choice.
Another important consideration is to use separate Tor circuits (IP addresses) for each coin or group, because the whole point is to avoid linking them together.

106437: <p>If an online store accepts crypto payments directly into its wallet there is a problem:</p>

<p><em>"Anyone can make a small transfer to any BTC address derived from the MPK so they can watch the store's and its client's funds as soon as the owner transfers funds from her wallet. I suppose possibly that the person who did this wanted to know if cracking the store worth the effort."</em></p>

<p>In a previous question a user replied that a solution to this is to use a crypto payment processor like BitPay but:</p>

<ul>
<li><p>How they handle wallet privacy? Isn't it the same? The BTC only gets
to make a higher number of transactions between different addresses
but still, they can be traced to the store owner's wallet and her
client's afaik.</p></li>
<li><p>Or if the transaction is too small to match the price of the
purchase, BitPay returns the transaction to the originating address
and the BitPay address that accepted the transaction gets dumped so
it can't be used to trace BitPay's customer real address?</p></li>
<li><p>Another way I think this could be avoided as well is if BitPay
converts the payment to fiat, then to BTC again and then transfer the
funds to the customer wallet.</p></li>
<li><p>Wouldn't it be easier just to avoid payment processors and convert
the BTC to a privacy coin like Monero? For example using a
transitional wallet in the store to accept payments, making smaller
transactions to an exchange, converting to Monero and back to BTC and
then transfer to the owner's real wallet?</p></li>
<li><p>Could this method conceal the store owner and her client's wallet
balance to any potential hacker who makes a small transaction to any
store wallet MPK derived address?</p></li>
<li><p>Or the only way is to use a payment processor? If so, then how they
conceal their customer's wallet balance?</p></li>
</ul>

<p>Thanks for your attention, it's a confusing subject for me and I hope to have written my questions as succinctly &amp; understandably as possible.</p>
 123812: <p>TL;DR: your question is a valid one; there is no easy solution but CoinJoin and XMR swaps will help.</p>
<p>The 3rd party payment processors use wallet that is shared for all customers so this specific attack against specific merchant is not effective. But then you have another problem: the payment processor itself <strong>will</strong> leak your data, 1) ~legally to multiple partners, surveillance companies, regulators and LE 2) accidentally due to security breach, ransomware, etc 3) illegally, the data will simply be sold at some point. So it's a big no-no.</p>
<p>So what are the options?</p>
<p>To start with, use open source, self-hosted payment processors running against your own node. Examples include <a href="https://btcpayserver.org/" rel="nofollow noreferrer">https://btcpayserver.org/</a> or <a href="https://cypherpunkpay.org/" rel="nofollow noreferrer">https://cypherpunkpay.org/</a> (disclousure: author).</p>
<p>Then either:</p>
<ul>
<li>pass coins through JoinMarket or Whirlpool</li>
<li>swap coins to XMR via near-instant, no-KYC exchanges (<strong>then wait</strong> because you cannot simply swap the same amount back to BTC)</li>
</ul>
<p>The important thing is to work with individual coins (UTXO-s) or very small groupings of these - just to fit minimal thresholds required by the privacy tool of your choice.</p>
<p>Another important consideration is to use separate Tor circuits (IP addresses) for each coin or group, because the whole point is to avoid linking them together.</p>
