Looks to me like you never paid the fee in the first place for the 1 item you sold. They should not be charging you that amount every month if you are not selling anything. Seems like they are sending you a bill for an unpaid fee that you owe them because the email says your account is overdue.
If you pay them the 1.55 you should not owe anything else unless you sell another item. If you don't pay it, of course they will send you another bill next month and they will likely suspend your account eventually.
That is how it appears to me unless you still have items up for sale and those are listing fees. Without more details it is hard for someone to guess what is actually going on in your case. Did you pay the fee for the item you sold?