Yes, it costs credits to do the same mod on a different car and also costs credits if you want to deactivate a mod you've installed. However bought credits are relatively cheap and it's not hard to accumulate free ones by watching a few ads. You're still well ahead of the cost of a VCDS.
Also, it's the macros that cost credits, not manual entry.
Wow. I did not realise that it cost you to deactivate a mod that you may have paid for in the first place. That sucks.
So if you wanted to trial a number of mods (to see if you like the behaviour), you will be burning through your credits.
Regarding the accumulating of credits, it sounds like how it works when you are trying to accumulate credits when playing some PC racing games...
You watch 5 ads and get 5 credits (or those keys), ie. Asphalt 8
Eventually, in a month, you have collected enough to perform another car modification/upgrade.
As it is when playing "Asphalt 8", this is very time consuming and I only do this in the background (on my laptop) while watching TV.
This is this reason that so many gamers go and cough up real money to buy enough credits to purchase a car (in this game), up to $120+. Big rort.
Getting VCDS may be more up front, but you are paying for the convenience and don't need to top up credits by paying extra or by spending time to watch a certain number of advertisements.
PS. My newly purchased HEX-V2 (3-vin) cable is going really well. I will do a review shortly of the benefits I have observed over the older HEX-CAN-USB cable.