need some help with different shipping carriers for particular items

I have seen some posts about similiar situations, but nothing concrete on how to go about it.

Heres my scenario.

I stock very few items. 99% of my items are drop shipped from my supplier via UPS. I am located in CA and have a couple items that i ship myself via USPS Priority Mail.

What I have been doing is that if one of my stocked items are in a sale, it adds the weight and i just eat any loss on the shipping charges.

What i want to do is if one of my stocked items are ordered by itself, the customer can just pay USPS Priority instead of the more expensive UPS. But if ordered with other items can just be the way it is right now and default to UPS. My stocked items are easy to calculate shipping for. 1-3 items can be shipped for $5, 4-6 can be shipped for $7, and 7-20 can be shipped for $12

any way to configure this?

it works perfectly if i use the "Table Rate" shipping method. but is there a way to configure it to only apply to particular items? and only if those items are by themselves? if other items are in the cart, i want it to default back to UPS