customer forgot to pay postage.

[red.p85]

How can they pay after payment for product already made?

[kopenhagen5]

Send the buyer a request for funds in PayPal for the postage amount.

[jellybirddesigns]

If it was the bamboo tray, it is listed as free local pickup, so you might want to change that.

The customer didn't "forget" - they only paid what you asked for in your listing.

[love_of_cars]

You can also list another item with the amount you need them to pay and ask them to purchase it to cover the postage.

[sandypebbles]

Have they told you they want you to post it, rather than pick it up in person, which you have offered as an option?