* if the item is expensive enough to be worth checking out physically, Amazon generally doesn't have it, since it's mostly full of Chinese copies and the genuine products are easier to ship from the manufacturers/smaller shops (since that's where Amazon actually has any margin)
* if the item is cheap, I'll gladly pay the margin in the physical store, since I'll get some extra joy out of having it now-now, not "when it gets there"-Amazon-"now".
I should note I am probably biased here from 3-7 days shipping times due to every Amazon order having to cross a border (into Switzerland).
So in this case you really have no reason to order the book from Amazon if it is available at the book store.
For other products this is however true, probably because the price difference and the sometimes cumbersome return policies of retail shops.
I genuinely want to support local businesses, and I make the effort to buy local from Waterstones whenever possible. But it's not always feasible with the amount I buy. I have many thousands of books, including those on Kindle. I don't have physical space for them otherwise.
However, for bookshops, I personally believe there's value in having a well-curated collection of books available locally, so I tend to allocate a portion of my book-buying money to physical stores.
An added bonus: Browsing for me is an excellent alternative to recommendation algorithms, and lets me pick up books that I otherwise wouldn't.
They may hate it, but $70 difference is too much. If it was like $50 I would have just bought the set while at the store...