It will combine and you'll be over your amount until you buy more slots or remove items; at least that's how it always worked before and what I've personally experienced. Same with character slots, except if you go over the amount of slots you own the characters will be inaccessible with a warning/notice/whatever you want to call it that you can't play them.
R.I.P. Elendilmir [30 Mar 2007-2015]
R.I.P. Ithil [23 Mar 2019-23 May 2021]
R.I.P. Anor [23 May 2021-10 Aug 2022]
R.I.P. my handsome Man Hunter [7 Apr 2008-19 Apr 2023]