Plugin that counts how many letters for customization

We offer yard letters, and have a plugin that requires shoppers to enter the text they want — however, there’s no way to count the letters they enter. Customers must add the corresponding number of letters to the cart manually. This, as you can imagine, ain’t working. They’ll punch in 20 letters in the field, and only add the min. 4 letters to the cart.

Frustrating for us and them. No fun.

What can I get, or code I can add to the site, that will count the letters for this? I understand Woo is NOT simple to alter and this is not an easy request. If Woo can’t do this, what other platforms can? We are not against the idea of putting this product on its own separate mini-site to achieve this functionality.

The page I need help with: [log in to see the link]