Hi @PayPal_Martin , I implemented the payment buttons as described in the docs and even had a detailed view at IPN / PDT functionality. My questions are: What is the best practice to send a unique identifier (which I generated on my server side) to Paypal which I always get back (regardless if PDT or IPN) to ensure I can identify the paying user? Is this the "custom" (cm) field or is there a better field to use (maybe something like e.g. token, hash)? In the designing process of a button I added some variables - but these variables are neither shown in HTML-Code of the button nor are they sent. What did I do wrong and what do I need to change? Thanks and best regards Eric
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