I think suggestions of asking for a discount are disingenuous. If the seller shipped you a left-handed bass, you'd say, "Sorry, I can't play a left-handed bass. Please refund and update your listing so the next buyer doesn't make the same mistake." If the bass is heavy enough where you can't comfortably play it, then it's the same deal. "Sorry, I can't play a bass this heavy, please refund and update your listing so the next buyer doesn't make the same mistake." It's not, "Sorry, I can't play a bass this heavy ... unless you give me some money back."