Maximum weight should be the lesser of the two shipping modules on how much they will ship in one box, unless you do not mind one shutting off when the weight is too much ...

The default is 50, for 50lbs ... if you set this higher and total weight of the order is more than 50lbs, USPS cannot ship it as the quotes will attempt to quote for a higher weight ...

Also, check your Tare weights as these add on for small packages, by default 3 lbs ...