The message "this product combination does not exist" appears when the buyer has selected a combination of Option values that you have not defined as a combination in that product's Variants screen. Sometimes this can also happen when the button code in your page has not been pasted-in correctly for the affected product.
For instance, on your page for Silk Laminated business cards, you have the Variants menus from the button code we issued to you for that product, but the button code's FORM tag (which has been stuck waaay higher up in the page source for some reason) appears to be taken from your UV Cards (#579333) button code -- those two products don't use the same Option values for their Variants, so submitting the Silk-lam product's values for the UV product results in a mismatch, hence "this product combination does not exist".
I'm guessing your page designer recycled an existing page from another product and forgot to replace that isolated FORM tag. This sort of mixup could probably be prevented by just using the button code exactly as we issue it and keeping it intact in one chunk in your page source, rather than splitting it up for whatever reason that was done.