Hi,
It is a really good idea !
The current system does not allow it directly but you can use a custom plugin and the triggers "onBeforeMarketCalculateVendorPrice / onAfterMarketCalculateVendorPrice" to override the way that the fees and the vendor price are processed.
I am currently making a big rewriting of the ordering system in order to add new features. I think it will be the best moment to improve the options of the fees.
So, in the current version of HikaMarket, you can override it using a plugin and in a future version, this feature should be implemented.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.