Once you have selected your items, click on the basket icon and then on 'pay securely', choose the address and the option to add a wicker basket appears at the check-out.
Simply click on 'create your own hamper - add a hamper here' and the section will open to show you the small, medium and large hampers available.
As indicated in this section, a small hamper takes 1-4 items, a medium hamper up to 8 items and a large hamper 9-18 items. According to the number of items you have selected, the hampers in which the items will fit will be available to select.