I would like to upgrade to Samsung S5 - however if I log in as a monthly customer, the price is $ 275, but if I am just browsing (without logging in as customer) the price is $ 0. Why is the price higher for a long time customer?
The Samsung S5 is retail price $635. If you get the regular Tab ($360, $15/month) it is normally $275 upfront, but it is currently on sale for $100 (in store). However, if you go on the Tab Plus ($504, $21/month) it is $0 upfront BUT the minimum plan (including tab charge) is $80/month.
The $0 price is for Tab plus pricing, which involves changing to a more expensive plan and a higher Tab charge. The Tab plan you're referencing when you log in is to make sure you keep your existing one.
But I can see a $100 price on the website with a regular Tab plan. Maybe you have an existing negative Tab that needs to be repaid first, and that's why you're seeing $275.