It sounds like this 129.95 for an engine flush is like selling a miracle cure to desperate people. If they would have changed their oil AND filter OFTEN, and preferably drain while the engine is hot, they would have NEVER needed this engine flush snake oil cure! If frequent and proper oil changes are done, there will never be the need for such scrubbing unless something happens causing extra filth, then it would be done AFTER the repair.
The main reason for frequent oil changes is that it is the only way to constantly keep the inside of your engine clean.
If the engine has degraded to a point of needing work, cleaning the inside of the engine, which frequent oil and filter changes would have done, is closing the prison gates after all the prisoners have escaped.
'84 Euro 240D
'88 300E 5 Speed
Over 800,000 miles in