From seeing threads about those sexed bantams, I would say you would be better off ordering a few extra straight runs and saving the money. You can always sell/rehome the ones that are roosters. It doesn't seem like their accuracy rate is 90% when looking on what people are getting when they order sexed bantams.
I am a little conflicted if the extra 6.50 for sexing bantams is worth it or if you should just order 2 extra birds for the same money. If you're willing to cull, buy the extra birds. If not, pay the extra and get a higher than average ratio of pullets.
I will say that MPC customer service is really very good and easy to work with.
I ordered about 15 sexed bantams from them 2 years ago. Only one was a roo. I was very happy with the Frizzles but the Silkies all died by 4 months old. I still have the Frizzles.
All my own chicks that I hatched at the same time did very well so I have never ordered from them again. I may order Frizzles again if I need to but never Silkies.
I ordered a total of 7 chickens from MPC. There was an original batch of 6 bantams- 4 silkies, 1 EE bantam, and a buff brahma bantam. 2 (the brahma and one of the silkies) died w/in 24 hours, and didn't look so hot when they arrived. I got excellent customer service on the matter, and opted for 3 Wellies as replacement birds. All birds were sexed. So far, I've been quite happy with the surviving birds, and nobody seems to be in danger of dying (although, you just never know). The birds are 5 weeks old on the bantams, 4 on the Wellies. I am beginning to get a little suspicious that one of the silkies is a roo, though. Obviously, it's too early to know for sure, so I'll just have to wait and see.
I would say that, overall, I have been quite pleased with the service I've gotten from MPC. If it turns out that I have a roo or two, I'm fairly confident that they will take care of me on it. It would be even better if it turned out that I have all pullets, but I'm not worried about the investment I made for the sexing. MPC has so far proven themselves to be good as their word on making things right, so one way or another it will work out.