Underpaid Postage?

Description

I have been selling and shipping and using Paypal for I would guess 20 years. I have NEVER had an issue with not weighing or measuring correctly. Today I get an email saying I owe for underpaid postage. This is a package that was sent and delivered at the beginning of December. They are saying the weight is correct but the dimensions are wrong. They are saying the package was 4 inches longer and 5 inches thicker than I claimed. There is no way. The BAG i shipped it in couldnt even stretch to 11 inches. Im just so confused. I already paid $16.34 in postage and they are asking for another $18.51. If I have to pay this I basically gave away what the person purchased. I messaged Paypal thru their system since I dont see a way to email, lets see how long it takes them to reply. I cannot print any postage labels until I pay this postage discrepancy. Any advice would be appreciated. Thank you!