There is usually a selection of 3 or 4 per section. Once you have chosen you text a number, it costs £1 and they will send you a text, pretty instant. There is a box to input the number they send you and voila, your box will be on its way!
I have been a member for a year, and have a few times received samples in addition to my chosen ones!
They often have deals on the full sized products of the samples they offer too. Although do shop around to make sure its the best price.