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Some Important Notes
The only way to really test your PayPal integration is to buy with another PayPal account. You could get a PayPal sandbox testing account, but it is usually beyond most users' abilities to set everything up (we also can't support this if you try it). So the best and easiest way to do this is to lower your price to $1 and have a friend/colleague buy with their PayPal account. You can always refund after your test.
Order Confirmation pages will not work with PayPal, so direct your customers to a Thank You page after purchase. The reason the Order Confirmation page type won't work is because a customer coming from PayPal to the Order Confirmation page will appear as a new visitor due to how cookies and page linking work.
Make sure the SMTP settings in your account (My Account > Settings > SMTP Email Settings) are set up properly in order for your customers to receive a fulfillment e-mail after purchase.
PayPal does not support One Click Upsells / One Time Offers.