Quote from Bitstream:
ja, that's what i wonder myself...never heard about one with fixed rates.
The reason why they can offer fixed rates is they will take your money away when you execute the order. Otherwise the firm will go bankrupt if most customers trade very large shares to take advantage of the fixed rates.
Some direct-access firms may offer fixed rates (beware of hidden costs or small prints). You need to calculate carefully to see if you can take advantage of these plans.
See http://www.mbtrading.com/ratesStocks.asp (It has a fixed-price plan, but a direct-access firm. Read carefully!)
Alternatively, you may try to find brokers with cheaper per-share (especially for lower prices)