My Macbook Air Usb-C port has stopped working randomly!

It's under warranty. Make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. Also, see How to find your nearest Apple Authorized Service Provider (AASP) or Distributor (AAD).

"Your Apple-branded or Beats-branded hardware product (“Product”) is warranted against defects in materials and workmanship for a period of ONE (1) YEAR from the date of original retail purchase (“Warranty Period”) when used in accordance with Apple`s user manuals (refer to www.apple.com/support/country). Under this warranty, you will be able to direct your claims to Apple even in situations where you purchased the Apple Product from a third party.” [Apple One (1) Year Limited Warranty – Accessory - For Apple and Beats Branded Products Only.]

I doubt that it would be the software unless the battery issue only happens with that program and no others. I do not know why the problems exist. There is no quick fix for a defective battery or for bad USB-C ports. You can try Reset the System Management Controller (SMC) on your Mac - Apple Support to see if that may reset the USB ports. Of course, I am assuming you are not connecting a peripheral that is overloading the port. Beyond that, I can suggest reinstalling macOS. You will need an active Internet connection to do the following:

Reinstall El Capitan or Later Without Erasing Drive

Please be sure you back up, if possible.

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility and click on the Continue button.
3. Select the indented (usually, Macintosh HD) volume entry from the side list.
4. Click on the First Aid button in Disk Utility's toolbar. Wait until the Done button activates, then click on it.
5. Quit Disk Utility and return to the Utility Menu.
6. Select Install OS X and click on the Continue button.

And, if the above doesn't help, then you can try this instead:

Install El Capitan or Later from Scratch

If possible, back up your files.

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility from the Utility Menu and click on the Continue button.
3. When Disk Utility loads select the target drive (out-dented entry w/type and size info) from the Device list.
4. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
5. Set the partition scheme to GUID.
6. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
7. Click on the Apply button, then wait for the Done button to activate and click on it.
8. Quit Disk Utility and return to the Utility Menu.
9. Select Install OS X and click on the Continue button.

I asked if there was a place in Finland where you could take the machine. Well, Euronics has shops in Finland:

Country Company Name Brands Branches

Finland Kauppiasosuuskunta Tekniset Euronics, Tekniset 63

I suggest you look into finding one near where you are living if reinstalling macOS does not help.

A pure exchange is possible within the first 14 days from date of purchase. Otherwise, the only other options are repair or replace (requires additional money.) Apple does provide a trade-in program: Apple Trade In - Apple.

Given where you are located, I think it would have been wise to bring two computers should one break down because you have no local repair options. That you have had no problems with a Windows computer is irrelevant. Like comparing apples to oranges. There is no evidence that suggests that Windows PCs are more reliable. PCs are made by hundreds of companies but only Apple makes Apple computers.

It's unfortunate that you are having this problem. You haven't indicated whether the battery will recharge when it falls between 10% and 20% remaining power. Generally, one should never run a lithium battery down to zero as that will shorten it's life or prevent the battery from recharging. You also have the AC charger which you can use when AC power is available.

Is there no place in Finland where you can get the battery replaced or tested?

Sorry to hear that's the case. Maybe you can rent a computer on which to conduct your business. Most cities have stores that rent/lease electronic equipment. If you can locate an Euronics (or whatever they are called in Finland) outlet, perhaps they can fix your Mac and rent one you can use for your business.

Does your computer work from the AC charger? In some cases a bad battery generates a short circuit that prevents the AC charger from powering the computer. Perhaps it will work for you.