Hi LCrowhurst
I'd be happy to explain the difference to you. Looking at the invoice, it looks like you had 2 customers use the online invoice feature. The calculation for the monthly surcharge is the total of - 1.8% for each invoice, plus a $0.25 flat fee per invoice.
For example in your situation, it is $0.50 + the 1.8% for both invoices.
So when you are looking at the surcharge amounts received, there will always be a difference between that value and the invoice you receive from us, depending on how many invoices used the service.
Please refer to Customer surcharging for online payments for more information on that topic.