payment problem - customer invoiced twice + no download link

Hi there.

I went live with my site on friday.
Im a little confused however as an angry customer emailed saying that there was no download link after he had paid using paypal. On top of which he recieved two payment notifications although only one payment was made.

I have made test payments using both credit card and paypal and it seems to work fine. Has anyone else exprienced issues like these?

Can anyone sugguest any reasons for this?

While im on the subject....

I havent yet activated my credit card payments yet as im not exactly sure what i have to do to accept credit card payments.

I read somewhere in the faq's that i need to setup a gateway.
Can someone please explain what this is and how i do this?

Thanks