Every company incorporates some form of drop off charge if you drop off in a different location. Some will show it as a separate local charge. Others will incorporate it within the total quote. For example, go to Avis and compare pickup in Lisbon and drop-off in Porto versus the same rental dates where you pick up and dropoff in Lisbon.
In the case of Zest, you do need to compare the different rental companies because they have different office locations. For example, if you pickup and drop off in Lagos, LagoRent does not have a local charge, but Rent Car 4 Less has an additional €130 mandatory local charge. That is because LagoRent's office is located in Lagos and RC4L is located in Faro. RC4L needs to deliver the car from Faro to Lagos and to return it afterwards. Also, it is cheaper to pick up the car at the respective Lisbon rental company office as opposed to Lisbon Airport, probably because airport authority may charge a fee or the rental co incurs an expense to deliver car to the airport or to shuttle people to off-site location to pick up car. It is all there for you to compare, or you can email Zest.
I find Zest very responsive. I will be using them for the first time this Sept. They have answered my emails within hours, cancelled one reservation, made a new reservation for a larger car and changed a delivery option so that I could pickup the car in the local office instead of incurring a €25 delivery charge to my hotel. Plus I can still cancel for free up to 48 hours before the rental start date.
As far as I can see, Zest includes the CDW and there is nothing else to buy or decline. Credit card CDW coverage always advise you to decline the optional CDW offered by the rental company. IMO, the CC does this so you do not waste your money. If you still opt to buy the optional CDW, your CC protection is still there, you will have double coverage, but you will not get double indemnity. Only one will pay out in the event of a loss depending upon the wording with respect to priority. When I rent in Hawaii, I sometimes have triple coverage: my home car insurance covers rentals in the US, my credit card has CDW protection and sometimes the Hawaii rental company or broker includes the CDW coverage.
I am guessing that Zest the broker (not the rental company) is providing the CDW protection for the customers that book thru them. Some but not all of the rental companies working with Zest require a deposit or hold probably to make sure that any damage is paid out immediately until Zest reimburses them, after which you would get your deposit back from the rental company. This is often what happens when you decline CDW coverage and rely upon your credit card protection. If you were to go to and book directly with those particular companies, they would often ask you to pay for the CDW or take a deposit/hold if you decline and rely on your cc. If you still have concerns, I suggest you email Zest and ask them to answer your questions.