The IRS (and really Congress) are the authority. Amazon lists items as FSA eligible so their purchase system accepts FSA cards.

> will I be reimbursed 99

Should be $99. You should request this amount and not the $100. In my experience FSA providers are lazy/incompetent. I had one reject my LASIK claim even though I uploaded the bill from my doctor that had all the billing codes and amounts. However, the bill had a boilerplate box that said "Insurance pending: $0". They rejected it because this evidently meant that insurance may have been paying and you can't double dip.