How do you add a note to a purchase?

[corred]

@shb_uk 

When you have made a purchase there is a box on the payment page to "add message to seller"

Please be aware that you cannot add any conditions to the sale here or ask the seller to send to a different address

[ruby*tuesday]

Why do you want to add a note to a Purchase?  There is no such option. Your Purchases are all in your Purchase History for 30 months.