Please note that there are some limitations of having Actionstep mergefields within an 'image' (ie a Pay Now button). Click here for some workarounds for this known limitation.
The RED text is what is provided to you by Quick Fee when they return the link to you. The GREEN fields are Actionstep mergefields and the BLUE text forms part of the link provided by Quick Fee, however, is consistent for each client.
Below is an example of a standard link that will pre-populate Quick Fee with the invoice number, invoice amount and the matter number.