Hi Jen, the financial institution the FSA is through won't allow the transaction. This is sometimes the case with HSA/FSA. So if you want to use the FSA, you will have to pay with a different card and submit for reimbursement to them. You will also need to get a Letter of Medical Necessity to submit with the invoices to the FSA. We have a 3rd party vendor we work with to get you a LMN which they will charge you $20/year for.