Having DPW calculate shipping could be a good feature, however it is not simple to implement, unfortunately. DPW would have to be integrated through software with each of the major carriers, such as USPS, UPS, FedEx, DHL, etc.
I say “could be a good feature,” because the buyer wouldn’t know the shipping amount until after they click the link to go to PayPal, sign into their PayPal account, and then return to DPW to confirm the sale. This is because DPW gets the buyer’s address from PayPal only at that time. This allows buyers to buy art through DPW without having to register or, if they do register, DPW doesn’t have to ask for their address, which would make registration more onerous and would potentially put off some buyers.
Instead, DPW allows member artists to set the shipping cost per work per region. While this may not be as precise, the idea is one should charge enough to cover one’s shipping costs. This amount can be calculated on the website of the carrier of your choice along with the weight and dimensions of the paintings you ship when packed.