In case anyone is still looking at this forum and trying to figure out how to get this contribution to work here is a little info I have found while trying to incorporate it into my oscommerce 2.3.1 shop.
Thanks to daoutlawz for his post, it helped me a lot:
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Once you are ready to go live, comment out that code and add this code underneath it:
$this->form_action_url = 'https://www.paymate.com/PayMate/ExpressPayment?mid=testuser';
where testuser is your username from Paymate.
After run a test payment through I put in the correct info for my paymate account and received the same message as dhindry
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Payment currency: *
Merchant is requesting a foreign currency payment and their account is not set up to do so
So I viewed the page source for the paymate payment page and found a bunch of hidden fields that it seems you can change by passing them values via the url. For example one is currency, so by changing the url to this :
$this->form_action_url = 'https://www.paymate.com/PayMate/ExpressPayment?mid=testuser¤cy=NZD';
I was able to change the currency to NZ dollars and remove the error message above.
I remember seeing another post where someone wanted to give the buyer the ability to change the payment value, you can do this by passing the parameter amt_editable with the value 'YES'
$this->form_action_url = 'https://www.paymate.com/PayMate/ExpressPayment?mid=testuser&amt_editable=YES';
There are other parameters that are listed there so have a look. I hope this helps those of you setting up this contribution.