I work in shipping. I'm not familiar with FedEx Smartpost, but we get a FedEx discount and One Rate prices provided we can fit our orders into the appropriate packaging. For us, it's $7-13 for FedEx 2 day (includes Sat. delivery for orders shipping on Thurs.) provided it fits in the right packaging. FedEx packs are ideal, but small or medium boxes work too (I believe the medium boxes are like 15x12x2 or 12x10x5 off the top of my head). Weight does not increase cost in those cases. For variable rate FedEx, it is more expensive and location and weight affect it. It is anywhere from around $15-30 for 2-day and $20-50 for overnight, and that's base weight price. It increases a bit per lb. up to 5 lbs. and significantly more at 6+ lbs., and if alternative packaging (aka not "official FedEx packaging"), it make increase even further depending on the dimensions.
USPS priority tends to be $6-8 up to a pound if using basic USPS packaging, and depending on location is generally 1-3 business days for arrival. "Regional Rate" packaging can be very helpful for packages that are of medium to heavy weight (as they depend only on location, not weight) and for especially heavy packages and/or far locations, their Flat Rate options can be even better (example: if you have a 10 lb. package being shipped from one coast to the other and it fits in a Medium or Large Flat Rate Box, you will pay more than twice as much shipping by "package," dimensions and weight than by Medium/Large Flat Rate, which are available provided you use the correct box(es)).
Generally, USPS is cheaper and more readily available. Disclaimer in this post: We get special deals for both because shipping-oriented business. We also get further discounts by using online services such as Shipstation and Endicia, plus we have FedEx, UPS and DHL all competing with rates >_>. To a normal person selling something online on a whim, this information will not reflect their options, but is meant as background insight as to how shipping works in general.
The bottom line: USPS will save you the most money if you are paying for shipping, and comparing package weight and dimensions against Regional and Flat Rate services is always worthwhile. You can minimize costs by doing this and the USPS is pretty much bound to it, so you have nothing to lose. FedEx has comparable options, but I don't think they are as readily available as with USPS. They have an Express Saver option which is 3 day (comparable to bottom-end USPS Priority), but it is limited. In general, FedEx is more restrictive and fickle, though on the delivery end is more reliable, in ETA and quality service. If 100 people ordered 100 things FedEx and 100 things USPS, 100 people will report that FedEx packages arrived in better condition than USPS packages by a large margin >_>
Anyway, if any of this is relevant and useful to you, tc, feel free to PM me if you have any other questions. If I have relevant insight, I'll share it :P