We use USPS because we get free boxes & tape on top of priority mail rates starting at $3.85 anywhere in the USA (which is generally delivered in 2-3 days including Saturdays) Also, it's easy to drop off at the local post office. Most of our orders are 2lbs or less and as we are a new small business we don't have a regular shipping schedule. It all works out good for us. When we checked into UPS/FEDEX & Airborne they all charge about $4+ "PER PACKAGE" for a on demand pickups along with weekly or monthly fee's for scheduled pickups. This can dramatically affect the cost of shipping for us. Also, they ALL have 'additional' fees for residential deliveries and rural deliveries which can kick your shipping cost up another $3+. That’s not even getting into 'dimensional weight' where a 3lb box which is 16x8x9 gets charged as a 7lbs shipment. -- this can be another surprise fee if you miss it.
Now, USPS 'does' provide a pickup service for $12.95 'PER PICKUP' regardless of the number of packages but we don't expect to use this unless we have over 30 packages going out (which then figures out to about $0.43 per package extra, not a huge deal) As for additional costs? They are none. Whether it's going to a business or residential address, in a city or in a rural area, the rate is the same. Also, the 'maximum size' for a shipment is beyond anything we send.
Now, you can print labels on the USPS.com website with postage and everything but the Stamps.com & Endicia.com services allow you to due this much quicker and easier along with including additional features we find useful.
Both Stamps.com and Endicia.com now have 'hidden' or 'stealth' postage which does not display the cost (similar to UPS/FEDEX/etc). However, Endicia.com will automatically paste an address into the label program when you copy one into your clipboard (no more manual entry of every customers first name, last name, address, city, state, zip and telephone number) After the auto-paste of an address it can then verify the address, city, state & zip with the USPS database and make corrections, if needed, automatically. It can also be set to adjust the case of the address which makes it look more professional as customers tend to use lower case and bad capitalization. It is also easy to select different services such as signature required, etc plus they provide online access to view reports, tracking numbers, etc anywhere that you've got web access which is a huge plus.
USPS is not for everyone, a lot of businesses don't like the lack of tracking information and 24 hr update delays. However, for our 'ground' deliveries we feel it's not that important. I mean we sell keychains - so where's the emergency

. We encourage that any order 'needed' by a certain date be ordered via 'Express' and shipped thru FedEx 2-day service.
Geez - I can ramble! Sorry this ended up being so long, but I hope it gave at least one person some additional insight into shipping options and costs.