Note: Omnibasis does not bill you for or add any fees to your labels.
With EasyPost, you will most likely be charged a minimum of $20 credit to your account when starting up. This credit goes towards generating your labels. It is possible that you did not see this step and later saw a $100 charge from EasyPost: that's what this $100 is for.
You will be billed for the labels when they are created (because you are always generating return shipping labels). This is when your customers place a return request and their return is approved. As soon as that label is created, EasyPost will bill you for the label.
This is why you always need to make sure you have a sufficient balance in your EasyPost account to be able to purchase labels as your customers request returns. You are able to automatically add funds to your account when a certain low threshold is reached.
What happens if your customers never use the label you were already billed for? You can delete return requests within 30 days to automatically request a refund for those labels.
Also note that you can always be billed only once for a label: if your customers reprint the same label they first generated from the portal or their confirmation email, you are not billed again incrementally.