The weight of coin rolls will get you every time unless you sell a lot to one buyer and can flat rate it
Go to USPS.com and compare rates before you mail. If you print your postage online (prices at PayPal are discounted), you can schedule a free pickup. 10 years ago I stood in line at the post office 2 or 3 times per week. Nowadays it's 2 or 3 times per year!