namgostar on "[Plugin: Simple Membership] Paypal button invalid"

For anyone that might be curious about a solution to the above question...

Even though Simple Membership has simplified the creation of a paypal button directly from the settings of their plugin, you can still use the "advanced variables" option.

Here is how:
https://simple-membership-plugin.com/creating-a-paypal-button-for-your-membership-payment/