For a bike shop, I would think you'd want a basic management system to add/track/relieve inventory, apply proper tax rates to sale types (i.e. parts vs labor), track cash and generate information for your bookkeeping. Depending on your size, this system would also be your customer database with CRM tools, service tracking and your payroll system.

You might look at software that auto/truck/motorcycle dealers use. These are typically ERP-like systems that cost several thousand per month but maybe they offer scaled down versions for smaller businesses like yours. Companies like CDK, Karmak, Reynold & Reynolds and DealerTrack come to mind.

All this said, I have to imagine someone has developed management software specific to bike shops. My wife did direct sales beauty products and there were management software vendors specific to that! Good luck with the business.