Handling quantities in HikaShop with other systems

-- HikaShop version -- : 2.5.0
-- Joomla version -- : 3.4.3
-- PHP version -- : N/A
-- Browser(s) name and version -- : N/A

Hi,
I am just cusrious how all of you using HikaShop are dealing with product stock.
I mean if you are using another ERP.
If for example your client is already using an ERP to send orders, keep control of product stock etc.
Now you want to use HikaShop as a web shop, so my question is how do you solve the problem with keeping track of produ t stock if you want to keep all products in the same place.
For example if someone orders a product in the HikaShop web shop, then how can you see that in the ERP?
And if a customer buy something directly outside the webshop, which is quite common for many companies and stores that has both a physical store and a web shop.
All kinds of suggestions are interesting.
Regards
Micke