UPS and FedEx don’t care about their thermal labels, they figure the amount of labels used for other services is so low, plus they buy in such a volume those are just a rounding error.

Now the boxes, at least FedEx will start to cut you off if you order a bunch of supplies and have a low shipping volume with them, UPS DGAF.

Every package is dimmed (dimension's measured) and weighed as it goes thru the sort system. If it is any different then described by the original shipping label then you will be charged for it. Also dimensional weight can hit which you can read about in the service contract. So many prepaid labels go thru the system set as 1lb and no dimensions. So fedex has to weigh and measure in order not to hand out its service for free when they say slap that on a 150lb box. I would suggest dimming and weighing yourself or just not using prepaid labels at all so you know the true cost of these shipments. I know this is no solution to your problem....I've just worked around it and figured I'd give you some facts.

Edit: also as far as packaging and filler for your packages...just buy it from Uline...way cheaper then fedex branded boxes and just as good of a product.