I use Stamps.com and have been for about 9 years. I have used ebay, Paypal MOS, and Click N Ship.
Out of the 4, Click N Ship is the worst. Pros: Not having to wait in line and buy postage at the USPS counter. Cons: No address importing from ebay and no First Class postage options.
Ebay: Pros: Much easier to deal with seller protection because ebay knows exactly the address it was shipped to. Comm Rates. Cons: Too slow for bulk shipping, you have to process one at a time. No ability to print with a thermal label if buying international postage (at least I haven't figured it out).
Paypal MOS: Pros: Able to print batches with multiple orders at the same time. Cons: I haven't used this in like 9 years but there used to be importing issues sometimes. And I wasn't able to print using thermal labels (that may have changed now). Also you were only able to import 100 orders and no more. So on Mondays I would have over 200 orders but unable to print them all out and sort them by category because they only allowed me to import 100. I'd have to finish those first 100 in order to import the rest of the orders. So that's bad for high volume shipping.
Stamps.com: (What I currently use) Pros: Aside from the Pros you already mentioned... No limit to the volume you can import and any time. They've retained their version of International First Class Letter service even though USPS did away with theirs. I believe they incorporate some of their internal shipping combined with USPS. The tracking isn't as reliable but the cost is much lower. Able to use thermal labels for any printout. Makes it easy to combine shipping and upload it back to ebay when buyers have multiple transactions. Able to send postage at Comm price to anyone.
Cons: Lately they changed their importing methods so it takes much longer to import (you have to click multiple times and I have no idea what "Retrieving Orders from Marketplace" even means, haha). Takes about like 5 minutes to import 100 orders vs before it took about 2 minutes. Their insurance company PIP is easy to file a claim but they never cover the s/h you charge the buyer regardless if your shipping cost was less. So if you file claims, you'd better hope it was for one that you offered free s/h to the buyer so the entire payment is covered. Scrolling down to print postage is slow if you have hundreds of pending orders. They need to be able to hotkey a letter... I.e. Press S and you automatically go down to the orders beginning with S. Sometimes freezes or crashes when you are importing too many orders. Might have to try multiple times to get it done successfully when importing 300+ orders.
I don't know anything about ShipRush. But the postage service all depends on your seller volume. Stamps.com is good for me since I do about 50-80 a day. I used to do about 150 a day and found that I needed something that can handle that higher volume and supported multiple logins well (2-3 different people importing/printing/and uploading tracking simultaneously)... Stamps.com wasn't covering it. Keep me updated about Shiprush on how it works for you if you try it.
