Problems with upper gears

I recently had my rear derailleur replaced along with new shifters on a 18 speed mountain bike.

Shifting in the lower gears is smooth and there is no noise when pedaling.

I can shift smoothly into all gears however, there is a "clicking" sound when I am in the upper gears.

The bike tech who did the work said he could not find a seven speed shifter and installed an eight speed shifter and told me he would make adjustments so it would work.

I have been told this.

"It is a problem, but can be fixed by also replacing the freewheel, but I definitely wouldn't be paying for any service from a tech who didn't know this would be a problem."

https://imgur.com/a/xF73i91

https://imgur.com/a/5VB6gat