Filed married jointly on return had to amend it they sent both of us separately a letter saying we owe the same amount in interest, do we each pay or just one of us?

So we filed married jointly on our tax return, later found out we had to amend it so we did end up having to pay back 300+ something so we paid it in full. 20 weeks later ( delayed due to government shut down) we each get a letter in the mail from the IRS asking us to pay the interest fees on the amount owed it's only a little over 7 bucks but it's the same amount for him and I but mailed as if we were separate parties. Do we each have to send in a separate money order for the interest or just send in one 7 dollar money order and be done with it...??? I know it's not alot but I don't want to pay the government more than what I really only owe because they sent us what looks like separate "bills" but we filed married jointly and NOT separately so why are they sending it separated. I also don't want to send in one 7 dollar payment and then later on get another bill saying we still owe another 7 bucks ( plus whatever time elapsed interest on it due to not paying it). Please answer ASAP as the bill is due before end of month. Wish we could pay it online but they want a money order since it was amended return.