If it's limited to one, rather than doubling why not at cost +50%.
It's limited to one charm at a time, so rather than being slotless (double), it gets a half again-maybe even a +25% since it's *just one*?
I'd say it would depend on the potency of the collars as well. I mean a collar of dex with a charm of cha wouldn't be that big a deal, but a collar of freedom of movement with a charm of evasion would be pretty damned strong.