Invoice for item randomly showing up

[sigo-3801]

I sold an item in the middle of january and already payed off the invoice amount for that(which was around 34.5 dolalrs). However, I just received another invoice, for around 30 dollars, simply stating "shipping fees".  I'm guessing this is the payment for the shipping label that I printed, but why did it come almost a month later? 

[releasethekraken_1]

If you use UPS or FedEx, eBay doesn't know the exact cost until the shipper tells them. The initial cost they show you is just an estimate.

[handyhandz4u]

That might be because you underpaid shipping fees, either by weight or size.