I would like to see a change to the Handling Fee that's added to Shipping Fee to allow negative numbers (with confirmation prompt when adding negative). Then, after doing this, that value would be subtracted from the shipping fee presented to the buyer. If that results in shipping fee of 0 or less, present "Free Shipping" to the buyer.
For example: Say I have a small item with a shipping weight of 2 ounces. The domestic 1st Class shipping fee for this would be $2.54 plus cost of a box (maybe $1.00 for box). If I could enter $3.54 into the field, the buyer would get offered "Free Shipping" if they choose the 1st Class option. Or, they could select option of paying an additional $2.21($5.95 - $3.74) to have it shipped in a Small Flat Rate Priority Mail Box (box provided free by USPS). Or, if item is too large for that box, $3.54 off the USPS $5.75 to $8.35 (zone dependent) rate for a 1 pound Priority Mail package.
And, the "Free" Domestic shipping allotment (or part of it) could then be subtracted from the International Shipping rate computed for a non-USA buyer to not end up charging International buyers significantly more for items which are sold with "Free Shipping" to domestic buyers.