On my payment form, how can I give them the option of requesting an invoice instead of using paypal option?

I want to do this on one form. Is it possible? 

Since you're using PayPal your customers will be automatically redirected to the PayPal payment form, there is no way to change this. 

If you would not like to redirect them to the PayPal payment page then you will need to use another payment integration, and then if you do not want to charge them immediately then you may follow this guide: How-to-Enable-Payment-Authorization

Otherwise you will need to remove the payment integration from your form, but do note that if you do it, you will not be able to charge your customers with your form, you will need to do it manually or request their payment info after. 

Please let us know if you have questions, we will be glad to help you.