The 150 euros is likely a customs thing because they are sending books from the UK into the EU. And since they are not charging for shipping, you can always divide your order into several smaller orders, assuming there are no books or sets which are individually over 150 euros.

The customs change at July 1st last year, everything is now charged VAT, I do not know how your country is handling it, but for a lot of countries, namely mine, receiving books from outside the EU can be pretty expensive, and slow and bureaucratic. It is not necessarily the bookdepository's fault and they are the non-eu bookseller which seems to be understanding better how to pay for VAT themselves so you get stuff without needing to pay VAT and fees again.

Be careful about ordering books from outside the EU. Otherwise, sadly, just check the within-EU amazon stores, or any local stores.

Or you know, ebooks.