Finding my collection code

[tis.uk1]

I won an item and agreed with the seller to pay for it in cash on collection (after inspecting it and comfirming it was what I wanted).  The seller is now asking for a collection code, which I cannot find.  Apparently, to get a collection code I need to complete my purchase, but when I click the green 'pay now' button (which I am not sure I should be doing as I have already paid!), I just get sent to 'order details' again!

Can anyone help me please?

[**peaseblossom**]

It's on the app which not everyone uses.   The seller should be able to mark as payment received and item dispatched next to the order on their sold page.