Customs charge to pay but no slip

Hello r/royalmail

I've had a package arrive from Sweden today. I can see on the tracking that there is a customs charge to pay, but I cannot pay it online (when I click it it tells me there are no charges).

It's at my local delivery office - will I be able to go there and get it? If so, what will I need? I won't be able to provide a bill or anything with proof of address as it was being delivered to my office, but I have another package with that address on!