How to buy different options of the same item?

You have seen this great listing proposing a choice of ten postcards of the Houses of Parliament in London, and you wish to buy four of these postcards? 

Your first reflex may be to use the "Quantity" button and add "4" to your cart.

If you use the "Quantity" button, you will get this error message on top of your screen:

What you must use instead is the required* option. It can be called "Choice" or something else, but the important thing is that the name of the option is followed by the asterisk.

Do not touch the quantity button, leave it to "1", and add one by one to your cart the option number (example: postcard number 1, postcard number 5, postcard number 7, postcard number 10) and click on "continue shopping" between each choice.

Proceed to checkout once you have finished selecting all your options. Do not forget to ask me for a revised invoice to combine postage instead of paying immediately. The invoice I will send you will show that the postage price has been recalculated, and a 10% off coupon has also been applied to your multiple purchase  (discount for bulk order).