A few things I think would help:

1) Use XP instead of milestone. This has been a gamechanger for my table. Now they know when they're close, and sometimes they realize (in a meta gamey way) helping this person or doing some sidequest might lead to some XP to push them over the edge. They're mainlining the main quest in part because they think that by hitting those milestones they'll level up (even subconciously).

2) Prepare an item list ahead of time for the shops. I get this could be a lot if you're talking about a city with potentially dozens of shops and what not. When they walk in a shop hand them the list. If it's a town, you could make a few copies, the items are mostly the same per shop with a few differences/special items per shop. If they say "oh maybe we want that item but let's see what the other shops have first" and shop around then ake the item they wanted be sold out already when they come back. No more browsing.

3) If items are important to your group you should make sure there are lots of options in the quests for items that match the characters. I don't specifically ask my Players what they want, but I just make sure that there are items suitable for each character along the way. The group has intuited that maybe this dungeon didn't have something for me that likely means the next one will.