A screenshot may help, but if you check the wording of what the add-on does, I suspect you'll find it's the free facility to roam - in other words, enabling your phone to connect to foreign networks. The clue is the phrasing "enables the full roaming experience".
As far as roaming costs go, Croatia is within the EE-EU zone thus £2.59/day is the fallback position, this single charge allows full use of your UK allowances for calls, texts & data - with a 50GB/month FUP for data.
That cost can be reduced with 7/12 day passes. There's 2 relevant questions for whether EU-roaming is inclusive within your plan..
1: Did you last re-contract since 7 July 2021? If yes, then your history before that is irrelevant.
2: Are you on an All-rounder, Essentials Plus or Full works plan? The website tool should ask for this. If you do, then the billing add-on is inclusive. If you don't, then you can choose the daily, weekly or 12day pass.
