We've partnered with Flex so qualifying customers can use their HSA or FSA funds to pay for Nundies Youth Pants. At checkout, choose Flex | Pay with HSA/FSA. If you need a Letter of Medical Necessity, just answer a few easy questions. A licensed provider will review your answers, and if you qualify, your letter is usually ready within 2 hours. Not seeing Flex? If you're in Shop Pay, choose "Checkout as guest."