Aria here, I'm on the Social Care team at Shopify.
None of our apps will tie you into a contract. You are always welcome to uninstall them when you don't need them anymore and this will automatically remove their permissions to charge you. The exception would be if the app somehow billed you outside of the Shopify platform. In that case, you need to deal with the app developers directly to cancel but they still should not be tying you into a contract.
If you have further questions about third-party apps, the contact details for the app developer can always be found on the page for their app in our app store.
Sure, you can cancel the subscription (just uninstall) for any app right after your sale will be finished.
Feel free to try our Bulk Discount app for your next sale!