When clicking on the vendor, you will get this window

The Discounts button informs you of the discount available for this product

The Gift button let you enter the name of the person you want to send a gift to. Once done, you will have to right click on the vendor, and select Pay to pay and have your gift delivered.

Clicking on Buy displays this window

Clicking on either of this buttons display this window. Go to the page, and follow the instructions. Be aware that you will have to pay a little transaction fee to use Paypal or a Credit card.

This button doesn't need too much explanation :) If you know you're going to buy for yourself in L$, then you don't need to left click on the vendor. You can directly right click and select pay

You should first wear your gift card before clicking on this button. The price of your purchase will be deducted of the balance of your gift card, and if you don't have enough balance, a new window will ask you permission to take the remaining amount of money from you.