Dainkes,
It seems your looking for a third party application that would allow to you to bridge the gap between your offline brick+mortar presence, and the online world. It is increasingly popular for many retailers to use their physical stores as both a B2C consumer facing store, and as their fulfillment centers for online orders. Additionally, I assume since you are selling clothes that you have a multitude of SKU's, which only further complicates the inventory situation. Without any new solution, your best solution is likely separating your inventory quantities to prevent against various issues. If you want to move to automating this, then I would recommend that you check out eCommHub (where I work) since we have experience bridging the gaps between POS+Web commerce, especially with retailers focused on apparel products.
Best of luck with everything!!!