Hi Frank/ Fabio
In the United state USPS offer flat rate shipping depending on the weight and size of the package and have many flat rate box and envelopes. Many re-seller us USPS
due to large saving on freight charge.
Example envelope up to 70LB $5.95
Small box up to 70LB $5.90
medium box up to 70LB $11.65
Large box up to 70lb $16.95
I would be a nice feature to have in Ready pro, to be able to select a fix shipping cost per selected item that will meets the USPS requirements