We use the "Quantity Options & Fix Prices" extension and it does exactly what we need for our site. Backend config is intuitive and provides ... show more all the features that one could need.
Support is exceptional! The extension needed a few tweaks to accommodate our custom theme and Peter was quick to sort it out and continues to improve the extension to iron out any niggles.
If only one could get this type of service from all extension developers!
Great modules. Great support. Best is that the MageParts is willing to help out and enthusiastic about their products. Highly recommended.