Once you have entered the "Shop" section, choose the brand you prefer and then select the value and quantity. Cards purchased on SixthContinent generally have a 2-4 day activation wait time. If you see that there is an "Immediate availability option", that means that you can choose to add a Sponsored Card. Sponsored cards offer you not only the immediate activation, but also lots of extra credits which you can use towards your future purchases on SixthContinent.
*Please note that not all brands will always have the option to add a Sponsored Card or immediate availability.