Update pricing before adding to cart

Hi,

how can i update the pricing of a product before adding to cart?

I bought the products addon from WooCommerce. The customer can enter text to engrave a product. 15 letters are free – each additional is e.g. 0.10. So i check the input box if more then 10 chars enter – if yes, i want to add e.g. 0.20 to the product before adding to cart. If i then view cart, the product is not for example $40, but $40.20.

How to do that?

http://wordpress.org/extend/plugins/woocommerce/