Hello

Does anyone have experience in preparation of herbal syrups for sodium benzaote determination on RP columns?

I would guess that the preservative levels are reasonably high. Have you considered "dilute and shoot"? If there are too many interfering peaks you could modify pH in the 4-5 range, around the pK of benzoate. The peak will move around to some degree. Be sure buffer concentration is high enough to be stable -- 20mm at least.
If you really want a method for messy samples, I recall a method from years ago, Florida Dept of Ag was testing orange pulp wash (knockoff cheap juice by washing old pulp) by first using SPE at acidic condition, then washing the SPE with basic condition, converting the suppressed benzoate to salt, eluting it quite nicely. They were looking for very low levels.
My rule, still, is don't do sample prep unless you've proven you need sample prep.