I have a dropshipper, so I don't have to deal with fluctuating shipping costs much, so my experience with shipping calculations are limited. But, you have choices at PayPal to calculate as a percent of order value, set rate, or by weight. I noticed at the USPS site they calculate by weight, I haven't been to the UPS site yet, but it is probably by distance, because I know they allow up to so many lbs. & package has to be under a certain overall size.

With more sophisticated payment processors; there might be a way to link to UPS & USPS to have them calculate the shipping cost, but I don't think we can do it with a "Free" PayPal account.