Difference between ついでに and たびに in the following example

Question

In Nihongo so-matome N3 grammar p53. In question I number 4 there is this question written where both answers seems fine to me.

おとなりのひとはりょこうの（a.たびに、b.ついでに）おみやげをかってきてくれます。

If we choose たびに the sentence will have the meaning of:

• "Every time my neighbor go on a trip he gets me a souvenir."

If we choose ついでに the sentence will have the meaning of:

• Since he was going on a trip he got me a souvenir."

Why is the answer たびに and we can't use ついでに? Is there something that I didn't notice in the sentence?

Answer 1

Grammatically, they both work. The only reason I can think of for ついで being wrong is a little nitpicky, but it does highlight an important nuance of this word.

ついでに is very hard to translate appropriately. The concept is similar to the “while you're at it” in “While you're at it, could you do this too?”. It is to take the opportunity to do something else, since you've already done something similar, or in close proximity.

• 薬局{やっきょく}に行ったついでに、コーヒーを買って{かって}きた。
  This makes sense if you are able to buy coffee on the way to/from (or somewhat nearby) the pharmacy.
• 薬局{やっきょく}に行ったついでに、薬{くすり}を買って{かって}きた。
  I won't say this is blatantly wrong, but it is a little strange in most situations. You usually go to the pharmacy for the exact purpose of buying medication. This doesn't really fit the concept of ついでに.

Perhaps this is what the textbook wanted to point out. おみやげ, by their very definition, are exactly what you buy on trips to bring home. In exaggerated terms, it's like saying you went to the zoo, and ついでに looked at some animals.

It would make more sense if there was a different noun in place of the おみやげ. For example, she gets cosmetics for you:

おとなりのひとはりょこうのついでに化粧品{けしょうひん}をかってきてくれます