I have 22 slots and I don't use the last 2 slots or if I want to use them I accept 3 different gifts and keep them in my order tab, so the buy extra slot is in another page alone and when I order I count my slots so I never go to the page where the buy slot is. I hope this is helpful.
But I've bought accidentally 2 boxes, the one when I was new in the game and tryed to tap the little i and the other later when the game lagged, I was tapping the screen and I came up with a Tulip Dresses display which is beautiful so I didn't mind so much. Now I tap very carefully and when the game lags I just wait and don't tap anywhere!