Since I bought a $500 GC, I received ten, $10 Mperks. The expiration date on them is 9/3. You can use them all at once or separately. The system will ask if you want to use one as your scanning. For example, as soon as you scan anything (even it's only $1) it will ask if you want to use one. If you say yes, the option to use the next one will pop-up once you've spent more than $10. As
@FoxC63 mentions above, be careful to watch what you have left to scan. There is no refund/partial use allowed. If you have $5 worth of stuff and choose to use a $10 mperk, that mperk is gone and you just lost $5 in value.