In the upper right corner of your Checkout menu, there is a box [1 X] which enables you to make large sales at once, just tap on this rectangle and type the Quantity you want to sell.
Notice that the button shows up the quantity you typed in the previous step. Now select the products you will add to your chart with this amount.
After that,you can continue with your sale. If you want to know how to make a sale personally, check our tutorial here.