SKULabs does not actually create any labels except for manual pickup / manual shipments. With the USPS for instance, you pretty much have to have a lobbyist to be able to create your own USPS labels. Depending on which provider you use for the USPS, you'll be working with Endicia, Stamps.com, Shippo, or EasyPost.
When we communicate with one of these providers SKULabs has to be online and your provider also has to be online for the request to go through. In some cases for carriers such as UPS both EasyPost AND UPS for instance have to be online and not "down".
It is unfortunately incredibly common for carriers or carrier providers to go down intermittently which is where software like SKULabs comes into play to fill in for these gaps rather than perhaps forcing the picker to wait 5 minutes while we tried until we failed on every package.
No software is immune to these intermittent downtimes which is why in SKULabs you can connect multiple providers and we automatically fail over when needed while you're using the "Cheapest" or "Cheapest available" method.