I'm using RT rates for UPS and USPS. I had a customer order a few items, one item was very large at 75lbs and marked ship separately. The others items were about 25lbs total. When the shipping screen came up the customer was able to select Priority mail at a very low rate as if it only calculated 25lbs of items. Priority mail caps at 70lbs and shouldn't have been an option. I switched ship separately on the 75lb product to no and tested a duplicate order. This properly caused USPS not to be available at checkout. This is probably an issue regardless of weight though as long as the product is marked ship separately. UPS works correctly.