I am looking at this issue.
one thing that you guys should know is if paypal or google is unable to process the payment or if the payment has a pending status the links are not generated until the payment is approve.
if your customers are not getting the email advice them to check their spam box.
Some hosting companies restrict emails created by php, your server should have a registered email address that looks like this [email protected]
One other thing, check is your image sizes make sure that you are using pixel for downloadable image and that the images that you are uploading are big enough.
Image sizes will work better if you name them using the image dimensions something like 20×20 16×26