Hey there Christopher!
Genevieve from Shopify here :)
To cancel a paid app you just need to delete it by selecting "Remove this app" from the apps option menu on your admin apps page, like so: 
Ordinarily you will find one final charge for the app on your next invoice. This is because, while you get charged for your Shopify subscription and apps in one invoice, the billing cycles are almost always out of phase with one another. As a result, you pay for apps partly in advance and partly in arrears each month. You can find a more extensive explanation of this here:
https://docs.shopify.com/manual/settings/account/invoices#app-fees
Hope that helps!
Genevieve S.