yep, those are the rates...
1st Class = $3, Priority = $5.15. Domestic only.
Really the only limitation on Priority is that I can only fit about 8 30ml bottles into one of those flatrate boxes. More than that and I look at packing the order into my regular padded envelopes and then putting those into a padded Priority envelope.
Unless the order contains glass bottles, then it has to be boxed Priority or otherwise. And it doesn't take much glass to force me to ship Priority even if you select 1st Class. As long as you stay under a certain size, then it's all about the weight.
It actually makes me smile when an order with 1 or more glass bottle is requested Priority. I can't use the small flatrate box for those (no room for proper padding), so I use one of my boxes or one of the USPS Regional Rate Priority boxes.