eBay offers Flat Rate Shipping and also Calculated Shipping. But what if you want your flat rate shipping to vary based on weight? Below Calculated, you now have the option to select Weight-Based Rate which allows you to set a base amount and a per pound amount. This can be set as a default in Settings > Defaults.

The shipping amount is still submitted to eBay as Flat Rate, but it will add the Flat Rate base amount with the rate per pound, rounding up to the nearest pound.