We decided to adjust our price to benefit more customers.
Now the price is $2.99/month and $29.9/year.
We made this decision since after calculating according to the overall usage of all users and overall fees we should pay to backend services, we found out that a lower price could also cover all the fees to keep the product running.
All paid customers would get an equivalent credit balance to be used for future billings. For example, if you have paid monthly price twice for $5 each, you would get a credit balance of $4.02(= $5*2 - $2.99*2). Hence, you need to pay $0 for next month and $1.96(= $2.99*2 - $4.02) for the month after next month. Then $2.99 each month.