How to set up PxPay Recurring DPSBillingID?

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Hello there, 

I am looking to set up recurring billing through Payment Express. We had this set up on our last website and need this set up on the new Shopify site. While Shopify says they support PxPay 2.0, we are hitting a roadblock when trying to send the DPSBillingId back with every transaction, in order to set up recurring payments. I am finding it unclear how to actually insert the suggested parameters in order to integrate... 

There is a description for how to set it up here: 

Has anyone else found an answer to this problem? I'm happy to hire someone to make this happen!