You can add a small amount of shortening to your SMBC to keep it heat-stable. Gretchen from Woodland Bakery uses SMBC and her recipe uses shortening and butter. I'd recommend high ratio shortening over Crisco though.
Based on reading that other thread, it sounded like things started a little rough. That's why I said don't be afraid to turn down stuff that's beyond your capabilities. Someone asked me for a carved Nemo cake last year. I said no way. I don't do carved cakes because I think I'd butcher them. After 15 years of baking I know what my limitations are. But given the questions on the other thread, it sounded like she was in over her head to begin with. Pricing was...
I should have made that more clear. As the OP is probably not running a legit home business, I just wanted to start with the basics since she was talking about what she spent personally on ingredients.
Obviously one would want to do the research regarding competition, market, etc., and of course take into consideration any overhead.
Perhaps OP would find some entry level business classes beneficial?
Why would you accept an official "order" if you don't know how to price? You can't just pull an arbitrary number out of the sky and hit the kitchen. That taps into your personal budget. It would be in your best interest to research how to price your cakes so this doesn't happen in the future. Also, if something is outside your skill level then don't be afraid to say so. Otherwise you create a ton of stress for yourself and potentially upset the customer.