Can I send people a receipt after they pay automatically?

I presume that you want to use the payment processing tool and then once the payment has been made to send them the receipt of the same?

If that is true, you could set up the payment tool following these steps: Setting Up Your First Order Form

Once you do, just set up the email autoresponder to fire upon their submission, by following these steps: Creating a Form Autoresponder

As you do you will have a form that will accept payments and email the details of the purchase right afterwards.

Do of course let us know if you have any additional questions and we would be happy to assist with the same.