1 If X is a topological (or metric) space and M is a metric space, then the set of all bounded continuous functions from X to M forms a metric space if we define the metric as above: d (f, g) = sup x in X d (f (x), g (x)) for any bounded continuous functions f and g.
Invisalign is orthodontic appliance, so since orthodontic treatment is allowed as an FSA expense, Invisalign is covered. Different companies have different rules concerning amounts set aside and the timing of how funds are used for orthodontics.
The Podiatrist will then be able to evaluate what you need and if the Podiatrist can accept an FSA payment will be able to order the product directly from the manufacturer. It could be that just an accommodative orthopedic shoe and insert is all that is required to alleviate your foot pain.