(UK) FedEx invoice for an unknown order, final notice issued.

Tl;dr FedEx issuing invoice for an order OP can't find any evidence of placing, invoice can't be paid until Monday but final notice has already been given.

Hello,

I am not entirely sure if this is the right place for it, and if it isn't feel free to remove this, but on April 1st of this year I placed an order from the Blizzard Gear Store (based in the US). I have done this before and am aware they use FedEx who there is often a considerable invoice for. After a while, I received the usual letters and eventually (admittedly with a bit of delay) I paid the invoice of £18.38 on May 16th. Now I've started receiving letters from FedEx regarding an invoice of £18.70 that needs to be paid. I haven't placed an order that I can think of, I've checked Blizzard and my emails along with my online bank account and aside from an Amazon order (Which is only of approximately £40 itself anyway) I can't find any orders I have placed from any site.

I know this is minor in terms of money but as a 17 year old hoping to go to university later this year, I'd rather cut back on unnecessary payments (and paying for things I haven't bought).

As their final notice arrived on a Saturday, I am unable to actually pay the amount to them until Monday, which is naturally wonderful. I have sent them an invoice inquiry but that could take up to 10 days to sort, and the letter states that "immediate payment" must be received.

Forgive me if this is seemingly obvious to sort, but this is fairly new to me and it sounds fairly urgent.

Thanks.