There are too many assumptions being made.
You say the first replacement lasted two weeks and add later that you think the system was flushed and the refrigerant replaced.
You say the second replacement (where the repair facility suspected the original switch was bad) you think was also done with a flush and replacement.
These assumptions cast doubt on the whole situation from beginning to end. If it was the high pressure switch, there would be no need to flush the system unless there was a major failure of the compressor which would dictate the flush and would require replacement of several other parts. (They'd be expensive as would be the labor, you would know this.) Perhaps the refrigerant was topped off due to a leak which puts the replacement of the high pressure switch into question. You don't know so neither do we. You may just have a small leak.
I don't know how Audi does it, but most manufacturers these days have the high and low pressure switches threaded into a port with a schrader type valve contained in the port. You should be able to replace either of these switches with no loss of refrigerant or an extremely minor loss that would not require adding any refrigerant.
You've fallen into a trap that is very easy to fall into. We all have done it. Because two HP switches were replaced, you assume the problem is with the switch. It's akin to taking your car in with a rough idle and telling them to replace the spark plugs. You suspect it's the plugs, they may be more than willing to replace those plugs even though it doesn't solve the problem. If you're unhappy with the results, you can only blame yourself. You told them what to do and they did it.
It's sounding to me like the repair facility is less than capable. My suggestion would be to try another repair facility. Ask them to fix your air conditioning. Don't tell them it's not working and that you think it's the HP switch.