ok, so, say it shows me I have 12 bp slots, and I equip the 'feeling fine' badge which says it takes up 4 bp, which would leave me 8 bp. but instead of 8, I have 4 bp left.

if I now put in, say, 'flower saver' (another 4-bp badge) I have -4 bp left, but I should actually have another 4 bp.

I'm gonna take a guess that somewhere along the line, badges charge twice the amount of BP slots. For instance 'fp drain' (1bp) takes up 2 bp slots, 'feeling fine' (4 bp) takes up 8 bp slots. etc.