1. Is the shop multiportal, several shops in one DNN instance?

Yes, Razorcart is multiportal so you can run several shops in one DNN instance.

2. Is it possible to show products or categories Cross Portal?  Yes

3. Discounts depending on ordered quantity?  Yes

4. What payment gateways are supported?

Authorize.Net and PayPal are supported out of the box.  Other gateways can be plugged in using the order pipeline.  See the following kb article for info and
sample project.  http://helpdesk.smith-consulting.com/kb/a1454/action-delegate-pipeline.aspx

5. Is the shop also suitable for European market (addressformat, payment etc) or primary for US market.

RazorCart can be used for the European market and any market in the world.

6. RazorCart requires DNN 8.02 and above.

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At your service,
Dave Smith