You can decide when your invoices are enabled to collect online payments by toggling the option on or off within your Invoice Simple account.
Tap the gear icon in the top right corner to access Settings. From here you should see a confirmation that your PayPal account is setup and connected, as shown here:
From here, tap Payment Settings and you'll see a screen like the one below, where you can turn the option for accepting payments on or off.
If Accept Online Payments is ON, each new invoice you create will include the option for your client to pay you through the invoice with their choice of credit card of PayPal payment. Invoices you create prior to turning this on will not include the payment option.
If Accept Online Payments is OFF, each new invoice you create will not include the payment option. Previous invoices you created when the option was turned on will not have the payment option removed.