This sounds strange...
You don't need any receipt. If there is an invoice following the package, thats what the customs have to work with.
UNLESS the customs people suspect foul play. Worst that can happend is the package beeing sent back to its origin.
Why did you get in contact with the customs anyway ?Why not wait an extra day or two and get the package through the mail ?
I used to do the same thing. Tracing my packages. Whenever I saw them arriving inside the country, I'd rush off to the customs to pick it up myself. Somehow the customs dudes did not appriciate that. After I stopped and let them do their thing, I haven't had one single problem whatsoever with this sort of thing....
Best advice I can give you is keep cool, don't rush it.
If the customs are hellbent on a receipt...transfer the amount to the seller...give the receipt to the customs...get the seller to transfer the founds back to you once you have the package