Cal here from Shopify, thanks for reaching out to us. I hope you're keeping well today!
You are correct in suggesting that the checkout is disabled for anyone who is on a paused plan. There is no workaround for this, and if you wish to test your checkout and your payment gateways, you will need to change to one of our active paid plans.
How is the rest of the store setup coming along? Are there any other difficulties that you have encountered in getting your store ready to launch?