Adding an extra fee to a product with the Product Add-ons plugin
Variable products are incredibly powerful and you can do a lot with them. In some situations where you have a ton of extras or add-ons, it doesn't make sense to make a variation for each one. In such cases, it's easier to use a separate plugin that WooThemes has created, called Product Add-ons.
We can add an add-on cost with the help of the following steps:
From the WordPress admin, go to the Products page and click on Edit for any one of your products.
Scroll down to the Product Data panel and click on the Add-ons tab.
Click on the New Addon Group button.
Type in the name of the add-on group. If you're selling frames for your poster, you probably want to call it Frame.
You can now add as many options as you want. Make sure you enter a label and price for each one.
Click on the Update button.
You now have plenty of options to choose from on the frontend.
There's more...
There are different ways to use add-ons. With frames for a poster, it might make sense to only let the user choose one add-on. In such cases, you can change the Group "Frame" setting to use Radio buttons instead of Checkboxes.