How to integrate Server Integration Method (SIM) to WordPress

I’m actually a freelance graphic designer, I sometimes make simple WordPress websites for clients—the client I’m currently working with is requiring a payment method for their customers on their website.
Typically this is easily done with something like the Simple Checkout method by (or PayPal’s version).

However, this client requires it to have a place where customers can input their invoice # with the specific amount of their invoice.
So the Simple Checkout method WILL NOT WORK. has advised that I need to integrate the Server Integration Method (SIM) instead and has only given me this to work from:

I’ve been scouring the web trying to see if I can find a step-by-step guide on how to integrate this but to no avail. So here I am. If anyone can help point me in some direction, I’d be very appreciative!

Read more here: How to integrate Server Integration Method (SIM) to WordPress

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