Aria here, I'm on the Social Care team at Shopify.
If you decide to enter your payment details and choose a plan during the trial, you won't be billed until after the trial is over.
What sort of things do you still need to achieve to get your store ready? Anything I can help with?
This guide is a good checklist to help you decide if you're ready for launch.
This is the same answer I provided above, you will pay after the trial is over.
@PipHic, typically we now keep your store data for 90 days so you should be able to retrieve everything you were working on if you choose a payment plan now.