Extremely fast shipping time

The use of these two possibilities is quite flexible. In the case you are mentioning, both will do, but the construction with "time" in the plural is a little more explicit. The plural would still be proper in the case of a single item, if, according to a process of delivery that depends on the location and the distance, times vary significantly. Of course, whatever the context, additional precisions should be made available, and that would clear any possible doubt people might have.

"As for" is a standard idiom (OALD).

as for       used to start talking about somebody/something
SYNONYM regarding

This idiom is used when changing subject (Robert- Collins French-English/,English-French dictionary); personally, I believe that there has to be a connection between the two subjects, as there is in the present context (questions about shipping are bound to be customer's concern); of course, according to these context details, the definition in OALD does not include the case when something/somebody is the first matter that is being talked about.