Welcome to the forums!
I definitely agree with everyone here, a bigger cage would mean they are far less bored, they exercise more so they stay healthier both physically and mentally from the stimulation. They need to be able to run laps in their own time not when you let them out of the cage. You can see the hundreds of testimonials written here about how much a bigger cage has improved the lives of peoples guinea pigs:
https://www.guineapigcages.com/testimonials.php
Since you're also new to the forums I would highly recommend thoroughly reading these guides as well, they have lots of good information:
https://www.guinealynx.info/healthycavy.html
https://www.guinealynx.info/medical_guide.html
If you need any help finding materials for a cage let us know! You can find materials to build your own C&C style cage pretty cheaply in a lot of places or if you have the money you can buy them straight of the cages store, just remember the bigger the better
. They just need to be able to run around like this:
[video=youtube;YUOEElsL6Is]https://www.youtube.com/watch?v=YUOEElsL6Is[/video]
And please don't believe anything the pet store told you, they don't care about the animals just the money. The stuff they sell as people have pointed out are dangerous, unhealthy, and sometimes even fatal. While not everything they sell is bad, do not trust them for care advice. Pet store cages are not big enough for guinea pigs. Their sizes are based on laboratory standards. Just think of it this way, while you'd survive living in your closet, it wouldn't be healthy or interesting for you. The same goes for your pigs. If you want them to do more than merely survive in a tiny space but also have a good quality of life, definitely consider upgrading your cage to meet recommended cage sizes, which you can view here: Good luck!
https://www.guineapigcagesstore.com/about-candccages
Good luck!