I am trying to setup PayPal Payments Pro. I have a PayPal Business Account. I click the Get Your Sandbox API Credentials, and I copy-paste the User Pass Sig. Then I put them into the fields, and Test Mode is on. I click Test, and it says:

PayPal Error: Security header is not valid. Please check your API Credentials and try again. Also please note that Test mode is ON!

My site has a SHA256 cert on it. What is happening?

I am starting my AbanteCart install and setup. I entered one product with the l-w-d 6-3-1, Length Class inch, and the weight 6 with the weight class ounce. In extensions I have the default_usps module, and the weight class is ounce and length class inch also. First Class and Priority Mail are turned on.

When I check out, the USPS options come up, and the weight says 0.37 (which is 6/16 of an ounce), but First Class does not show up, only Priority Mail.

Why doesn't it show up?