How much money does the UK post office charge a person to receive a package?

My girlfriend lives in the UK and I live in the US and I sent her a package and timed it to arrive on her birthday. The last time I sent a package a year ago, they charged her to receive the package. This does not happen in the US and I'm not sure how much she is going to have to pay to receive the package. I believe they charge her a tax or duty according to how much the merchandise in the package is worth. I had to fill out a "Customs Declaration and Dispatch Note" declaring the value of the goods inside and it is my understanding they use this amount to determine how much she is charged. If I valued the merchandise at $27.75, how much will she have to pay to receive the package? Also, what if my estimation is wrong and the value of the goods inside is actually worth a little more than what I declared, will they know and if they find out, what will they do about it if my estimation is wrong? Please help me with these two questions, thanks.......