Hello,
Thank you for your feedback, we will take it under consideration for future improvements.
With the multi-currency system, such option might be complicated to implement ; we will see what is possible.
In the meantime, you can add a notice message within the "ordermarket / request" view thus your vendors will know that under a specific sum their payment request won't be taken.
Payment request is mostly an email sent to the admin, so the effects are limited.
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.