You need to contact your HSA custodian as soon as possible (like before April 18th) and ask them to treat the $800 as a Mistaken Distribution. Be nice because they don't have to do this. You will sign a form (it's probably already on their website) and send them a check for $800 (or whatever the exact amount is). For their part, if they accept the Mistaken Distribution, they will send you a corrected 1099-SA.
When you made the purchase, I am surprised that it was not flagged at the cashier because it was not for a medical expense. Please do the correction above as soon as possible because HSA custodian hate to fix things after the due date of the return.