I'd definitely recommend PGA or another strong alcohol if they don't sell PGA locally (some states don't).
Sometimes soap and water just isn't enough for really strong flavors (like menthol) and plastics can carry flavors for some time depending on how long they've sat and the type of plastic the bottle is made of.
I normally mix 50/50 water + PGA or vodka, shake and leave it to sit for 48 hours (tends to help leech any potential flavors out). I'll then get a pot of boiling water setup and drop the filled bottles into the water and let them heat up for 5-10 minutes, pull them out to cool and then rinse with soap and water. It's a lengthy process for me, but I want to make sure they're clean and clear of any flavor.
Just a not, HDPE bottles (normally clear and hard plastic) do not fair well in boiling water. They will melt/shrink. I've not had any issues out of LDPE. They keep their shape well and I've nuked them in the microwave, boiled them and even had them near a hot stove eye (by accident) and they kept their shape well. Just wanted to give you a heads up because if you boil HDPE, you'll end up having to toss them.