We started doing this a year or two ago. We collect postal code (not full billing address) and pass it through to a vendor that calculates how much tax needs to

We started doing this a year or two ago. We collect postal code (not full billing address) and pass it through to a vendor that calculates how much tax needs to be collected. The vendor also calculates for us how much needs to be sent to which tax collecting entity at what times, which is handy. We’ve always collected billing postal codes from our users, so we didn’t have to worry about starting to do that - we just sent them an email informing them that at their next renewal we’d begin charging sales tax. I think for REALLY old subscriptions where we didn’t have that info we may have just treated them as being in non-taxable locations, but don’t quote me on that. You might want to consult a tax lawyer. We measured the effect on renewal rates. It was difficult to tell how many cancellations were due to the tax vs how many due to being reminded they had subscriptions they hadn’t realized they were still paying for (our interpretation leaned toward the latter).