When the deal was done we took measurements and a wild guess on weigh.
Isn't that your obligation to do so, though? Sounds like it's
your fault that you took a guess on the weight of the package and thus shipped it, misinformed. Considering you're saying that the package was cards, why not measure one and then multiply? If they are different weights then apply the same thing for each one. It isn't difficult. An 8 year-old could do it.
In factg we doubled USPS weight, but USPS would not accept the packages
Was the weight double of what you guessed or was it half of what you guessed?
so I had to take then to UPS, where the weight was WAY off. One package was over $50 and the other over $60, We had guessed the weight at less than 40 lbs total. When I informed the buyer, I got an "oh well".
And they forced you to ship it out? Did you not want to ensure proper communication and that the buyer was going to pay for the shipping instead of doing what you thought would have forced him to pay more for the shipping?