I think as it stands right now you can only force the sort, not designate a particular carrier by total sales order. I'm hoping for more improvements in this area myself

. Insurance wise too (especially between UPS/Fedex compared to USPS where you purchase it separately). With Fedex/UPS the package is insured automatically without additional fees for up to $100, whereas with USPS they have to pay for insurance. I wanted all my packages insured going out...so I added the insurance cost into the shipping and then made my insurance breakdown at the .60 Fedex rates I think...
Thinking out loud for your issue - maybe setting up a flat shipping rate? I suppose you could always add a checkout question to see if the customer has a preference for carrier with a note that states "we cannot guarantee your request will be honored, but we will do everything we can..." - but I'm not sure how to make that connection easily if they decide on the carrier that requires more money from the customer due to weight or area. You don't say what type of products you have - but if you have multiple weights, then this may not be the best option for you. Aside from that you can add a shipping fee within the product detail (the drawback to this would be that with EACH product a customer orders they would get hit with an additional.25 or whatever you set it for).
So until we see more improvements to the shipping section of 3D, may be the best get-around would be to do a mark-up (you can enter a fixed amount or percentage) for USPS carrier and leave UPS and FedEx on the realtime rates with no mark-ups. This way it's an overall fixed dollar/percentage on the shipping (not tied with adding a fee for each product ordered), which may be enough to cover the difference (especially if you would know your min & max weight rates you're working with).
Have you questioned support? They may have another idea that's staring us in the face

- if you learn anything different - please share!
J-Lynn