Enable authorization and capture - if you need to capture payments in order to change the amount later, change this to yes. Otherwise leave it set to no.
Custom variable - Optional variable passed back via the instant payment notification variables. EzPPb uses this internally and thus will append its internal custom variable.
Invoice number prefix - this is a passthrough variable used only with IPNs. PayPal does not allow duplicate invoice numbers and so a |uniqueid is attached by EzPPb to prevent duplicates.