Standard vs Priority shipping

How can I set up where my customers can choose between priority and first class mail for their shipping? I'm basing it on weight...

Thanks so much!

That would be a setting in the USPS module.

will this work with light weight items? The average weight of my items is about 5oz with the packaging... but when I put in the listing that it's 5 it's 5 lbs, not oz and from what I've read there's no way to change that. (Some are even less than that, I have some that go down to 2oz)

One ounce is 1/16 (0.0625) pounds.

When you use the USPS shipping method, the USPS quote returned will automatically exclude First Class Mail if the weight of the shipment is over 13 ounces (the maximum weight for 1st class). Just enter your items' weight accurately (and make sure to account for the weight of the mailing envelope you're using for First Class) and the USPS quote will handle it for you.