It's a collection code that eBay send to your phone but it's only for the app. Obviously if you don't use the app you won't get it but the seller can still mark as payment received and item dispatched on a PC or laptop.
Tell them to go to their My eBay and it's via More Actions, right of item on their sold page. Or on seller hub they go to Orders for that item, click/press the item number, scroll down to Sales Status and notes. Tick the date that they were paid and select from a dropdown box if it was collected etc..
