Thank you for reaching out with your question. The free trials offered by many developers for their apps allows you to try out and use these applications without making a payment, and removing the app before the trial's expiry means you will not be charged for using it during that period. However, if the app is still installed in your store after the trial ends, you will see a charge for usage of the app beyond its trial period in your next invoice.
I hope that clarifies things but please let me know if you're unsure on anything and have more questions.
Victor | Shopify Social Care