For postage, just go to a USPS "Automated Postal Center", select a size, place the package on the scale, select a package, zip code, delivery, and confirm. It may take 5 steps but may only take 30 seconds per item. Pay with your credit card, attach the printed postage labels, and leave your packages with the USPS.
If you use the Dymo just for printing address labels, then not only can you start with the cheapest label maker, but you can use the address label for any purpose.
For many daily shipments, you can use a Dymo Label Writer with combined address/postage labels customized for Endicia.
Endicia's API, unless they changed it, is an always-on program that checks if a certain file has shipping information in it. If it does, it prints out shipping labels and clears the file.
Endicia has excellent support; I believe actual developers answered your questions last I used them.