If you can figure out the approximate weight and size of box needed, you can mess around with the pricing calculator on USPS and other services to get an estimate. Try a few zip codes that are further away from you and that’s pretty much going to be your max price. Go from there.
If the two boots are $80 new, depending on condition I might figure on getting $30-50 for the pair unless they’re in almost perfect condition. Then add the estimated shipping to that. The buyer might pay you $50 shipped where you end up with $30 in the end if it goes across the country or $40 if it sells closer. It can be a balancing act. Sometimes if the shipping cost is crazy (heavy/bulky item) you need to adjust your expectation of how much you’ll end up with. For some items it does end up working out better to sell locally or just place at a consignment shop.