buyer bought new shoes with FREE Shipping, correct paid postage

After it's delivered - demanded me to pay $5.03 more for "additional shipping cost which she was charged by postman" - supposedly she gave the CASH without any receipt, or proof of payment
Provided photo of "due" stamp and hand writing $5.03 on the package, not USPS receipt

What the USPS says:
If postage is incorrect - parcel goes back to sender
USPS workers never get money from customers not at post office, just at door - and HAVE to give the receipt

I don't know what the trick it is - so BEWARE if you don't want troubles