When you open the cash app, click on the cash card (2nd one from the left on the bottom) and then right under your card it will either say explore boosts or swap. Click on that and then scroll through your boosts and see if you have the one for
Sams Club. In my cash app, it is one of the last ones. Make sure to read the details - mine says maximum discount of $20, valid once every 24 hours, online only. Once you have set the boost you can just go to the target website and place the order using your cash app card. It should immediately charge you 5% less than your total.
If you want to make sure that it is giving you the discount, make sure your cash app balance is a little less than the price before the 5% off. So for example, if you were going to buy 3 of the $75 Disney gift cards at Sam's which are 71.98 x 3 = $215.94, you should only be paying $205.14 with the cash app. So make sure your cash app balance is less than $215.94 (but at least $205.14). That way, if it is not doing the boost correctly, it will just decline the charge.