how to sum entries from the last entry to some entry in between dynamically?

hi,
I need some help in creating a sql.
the problem is as below.

assume that:
I am a store keeper
and I have the list of customer(user_id) transactions in my order table.
schema: Orders(order_id, user_id, amount_paid, create_timestamp)

I want to give discount of 10% for the customer who made orders of worth
(sum) at least $500 in the least time from now (i.e last few days, but NOT
ALL TIME LEAST TIME TO BUY $500 WORTH GOODS).
and 9% discount for the customer who made $500 in second least time from
now.
and so on

there is no time constraint.
it customer could take any amount of time to make $500.
the customer might not even made $500 bill till now in that case I should
get the first date.

the result can take upto the first entry time of the table itself.

in short its like sum up entries of each user in order table backwards (i.e
from last entry to the first) and find the entry that has sum > $500.
If there is some user who didnt even make 500 till now in my shop return the
first date of transaction/order.

can anyone help me on this?