Thinking about taking a heavy item that only fits in a larger than standard size bag on a Spirit flight.

Looking at luggage charges: I have to a) pay the checked bag fee ($51), b) the overweight fee ($125) AND the oversize fee ($150). Is that really right? Or will the oversize fee also include the overweigh nature of the item or vice versa? Paying >$300 for a checked bag seems egregious, even for Spirit...but if them's the fees, I'll just ship it.

Thanks!
tb