I may have figured this out.  In your Settings click the Accounts and Settings tab and then click on expenses tab in menu on the left.  Make sure "Make expenses and items billable" is turned on.  After your receipt is uploaded via Receipt Snap you can open the expense and make sure "billable" is check and enter the Customer you wish to send it to.  When creating an invoice for that customer a menu pops-up on the right asking if you want to add that expense to your invoice.  When you add it, the image automatically attaches to the invoice.  Hope this works for you

SB