Ace of Spades, I have moved many, many loads of steel over the years and I have dealt with this same issue. As they had signed the BOL as received in apparent good condition and 86 days have elapsed since then, I can't help but think that their just trying you on for size. If they seem to be absolutely serious about this, then you may rightly conclude that they might be in financial trouble and will try this with every load they've received since June. It may cause you do update your credit report on them to look for signs of longer and longer payment times. I have only one raw steel customer left and like every other steel business, they're seriously hurting right now. Even though they generally pay within selling terms, because of market conditions, I have them on COD only and they are mad as hell - better them than us.
About credit reports - my issue with the reporting agencies is that they do not differentiate payment histories to tangible goods suppliers and service providers. If you discover a variance in the report of some being paid on time and some not, you may conclude that service providers get paid slower than goods suppliers, and recosider doing any more work for them.