I use Fedex Prepaid labels for the time savings (no need to wait in line at the drop off point) but many times the actual charges on the Invoice are different from the prepaid label. Turns out, Prepaid is just a "courtesy" quote and Fedex can charge whatever they decide is the real price.
Of course, this is done without my approval. I had an item recently shipped that went from $63 to $112.
The Invoice Dispute process has ZERO information.
The lack of explanation on any Fedex decision is an appalling and arrogant business practice. Time to switch to UPS.