Hi,
There is no way to fix it because it's not a bug.
What you're asking for is a "feature request" and that request is already in the HikaShop TODO list for the "user points" plugins.
We can't provide any schedule for his implementation but we are fully aware that letting the user choose the number of points he want to use will be very interesting.
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.