As far as I am concerned, as a native speaker, for your example, I’d rater say « Je ne vais pas souvent au bureau de poste maintenant. ».

When in Paris area, I very rarely hear guère in casual conversation but I believe it happens more when I’m in Southern France (in Provence at least) where you can still hear, possibly because the provencal gaire was very common :

Je vais plus guère à la poste.

Here are some alternatives:

user2233707’s Je ne vais pas souvent au bureau de poste maintenant (formal).

J’y vais plus trop, à la poste.

J’y vais plus souvent, à la poste.

Je vais presque plus à la poste. (Thanks @LaurentS)

Ça m’arrive plus trop (or plus beaucoup) d’y aller, à la poste.

Beware that the way you identify negations would be useless with informal spoken French (like in my examples) because the ne has essentially disappeared from it.