Hi Manly,  
 I see you don't have your private gallery link available on your shop page. If you go to your administrative settings-look & design-store layout and customization you can tick the box to activate your private gallery from your shop. This way you can see your card with the shopping cart and proceed from there.  
 I don't do the suggested way of adding the card to my cart, etc. When a custom request is done, I go to my private gallery from the link on my shop page, click on the card to bring it to it's own page, copy the URL and paste it into the custom request thread letter to the requestor so they can just click on the link and take it from there. So far this method as always worked well for me.
I hope I have understood your problem and was able to help!
Take care,
Betsy