Is 'with' correctly used?

You may find it in informal or sloppy or informal speech, but, as I have said before, strictly speaking one is amazed "at" or "by" things.

Many native English speakers get prepositions wrong. For example, many people say they are "bored of" something, when they should say that they are bored with something. (One can be bored with a person or TV show or film, i.e. one has a feeling in oneself, or one can be bored by them, i.e. an experience or series of experiences was tedious.)

(In French, we have soirées glauques!) (But I digress)

I'll tell you a funny thing... In France when a woman has her monthly period, it is often called "Having the English to stay", because it is a dull, painful time when no fun will be had...

First the phrase "all the selection" is wrong. It should read "amazed with the selection".

I think I would use the preposition "by" in each case.

I am amazed by the selection available.
You will be amazed by the choice offered.

(actually I think that the plural for choices is better (assuming I understand the meaning of the sentence correctly)

You will be amazed by the choices offered.