Hello Jynks -
our shipping rates are competitive with other online shipping rates, which tend to be better than those of USPS directly. Unfortunately, there is no specific percentage discount amount we can provide you with vs. another service as there is some variation. I can only recommend that you check for an item you have sold using the Calculate Rates button on the Shipping Label page for an item on your eBay sales page and compare that rate with any service you are interested in using.
While I realize that this is not the specific answer you are seeking, we are unable to provide such a specific answer regarding the pricing of our shipping labels.
Apologies for any inconvenience this may cause.