Do I pay on both stores?

No. As of February 2020, for both the Pay as you go plan AND the Unlimited plan only the Source store is charged for copies.

What if I have multiple stores I'm copying to?
While the price DOES increase for each additional store you are copying to - we still only charge the original, source account for all of your copies. 

"How do I know which plan to subscribe to?"

Deciding which paid plan you should subscribe your stores to is dependent upon your use-case and intentions with Replay:

If you only intend to make a one-time copy or intermittent copies, you should subscribe any and all stores being copied to/from to our Pay As You Go Plan.

If you intend to make many copies, wish to setup a continuous, ongoing copy, and/or wish to copy Product Metadata, you'll need to subscribe any and all stores being copied to/from to our Unlimited plan.