I think your wife's reaction is what's known in counselling terms as a 'presenting problem - the issue is not that you bought chocolates, it's that your wife feels this is indicative of something deeper.
You need to sit down with your wife somewhere in public - a restaurant is always good because it can't decend into a screaming match, and ask her why she was so upset. The key is to loistenm to everything she says without interupting.
Tell her that you understand why she is upset, and that you deeply regret having caused the argument. Tell her that the purchase was on a 'c.o.d.' basis, and not as a gift, and is no different from the shoe polish for your male colleague.
Tell your wife that if she'd prefer you not to bring gifts under these terms, you will comply with her wishes, and then reassure her that you love her and she has nothing to worry about.
On no account try to minimise your wife's reaction - telling her she is over-reacting simply makes her feel foolish as well as hurt - and that's what this is about.
Why not surprise her with a weekend away, and spoil her rotten? It may be that your absences breed insecurity - how ever ill-founded. Remember, you know you are faithful, your wife is feeling some doubts. even though rationally she knows you won;t stray, emotionally she feels vulnerable, and this incident is the trigger that has released her feelings which she has bottled up.