Hello, currently we use fixed recurring subscription billing agreement (for example user agrees to pay $20/m). It's a problem if user wants to upgrade or downgrade a plan, as we have to cancel it and then user has to subscribe again. I noticed some merchants are able to ask me just for "billing permission" and then they can bill me for any amount at any time. What kind of billing agreement is this? Are reference transactions required in order to achieve it? i
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