How do i make my product variations appear on my invoices?

[b.p.games]

Hi, can anyone tell me how I make the customer selected product variation, appear on the final printed invoice?

For example, the listing "Red T-shirt", available in S, M, L or XL. If the customer chooses say M from the variation menu, it doesn't show this on the invoice I can print off, it only says "Red T-shirt". 

Is there an option im missing?

Thanks in advance.

Dave BP Games