It looks like they've slightly improved the features and dropped the cost more on their followup model https://www.co2meter.com/products/co2mini-indoor-air-quality... this is not going to be lab grade accurate but if it works as well as their old one it's great for the price and fits your requirement.
Important is that you can adjust the automatic baseline calibration duration if you operate the sensor in a closed space that does not regularly get fresh air. Ideally put it into a longer period like 30 or 90 days or switch off and manually calibrate it regularly. I believe not many monitors offer that flexibility out of the box.
If you have some basic electronic skills, you can actually look at our open source build instructions and just build the basic monitor with the CO2 module [1].
https://www.amazon.com/dp/B07YY7BH2W
I bought one for myself, fine, then two for my siblings. The two I bought them always measured about 100 apart, when right beside each other.
To say – use the measurements as a comparison of what you know.
They have self calibration routines that assume they see atmospheric CO2 levels at least once either per day or per week
If it's always inside that assumption is wrong and it will always report lower than true CO2 levels.
.. pretty much all instrumentation requires semi regular calibration, it varies by how often and how serious you are about exact results.
Many CO2 meters are sensitive to dirt | dust | crud build up ( see article ) and should be reset semi regularly as per the manual (or by the linked site guidelines if you've gotten a cheap one with no documentation).
PM I have something that shows air quality and it shows 0 and high at the same time as the Qingping
- AirThings View Plus
- Sensiron CO2 Gadget https://www.sensirion.com/products/catalog/SCD4x-CO2-Gadget/
- Kickstarter: AirSpot https://www.kickstarter.com/projects/airspot/airspot-the-per... (uses the Sensiron sensor)
Maybe this isn't enough for you, but I know it's easy for me to get trapped in a rabbit hole of how to solve things in a specific way and at times I need to pull back and say "what am I actually trying to solve". I had this with co2 monitors, getting fairly into which one to get then I realised I can deal with this by just regularly opening the window. I don't need a specific number.
edit -
Air exchange may happen pretty quickly, depending on the room and surroundings, so it could be as easy as "every X minutes we take a moment to open the door and window".