Depends what your online store produces.
We're currently ( 1st Nov 2011) working on creating web service API's but freelock (a frequent visitor of the IRC channel) is already doing it. He doesn't mind sharing his code.
Web service API's discussion is ongoing on the development mailing list. Join and contribute
From the users mailinglist (2011.05.22):
Nigel Titley has an operational bridge working between Oscommerce and
LedgerSMB. He is happy to throw into the pot, especially once the
Ledgersmb RESTful interface comes out of the woodwork. It transfers new orders, new parts and new customers across from OSCommerce to LedgerSMB, handling multiple currencies and it updates the OSCommerce inventory from LedgerSMB. It runs as a regular cron job.
From the users mailinglist (2012.03.07):
LSMB integration with e-commerce?
The faq: REST API or other language-independent interface ?