Price difference for the order

sometimes, the order need to add some thing or change something, just need to make an new order here
Like add $14 USD, it need to make an order 14 items.
If the pirce mantissa is less than $0.79 USD, don't need to pay for the mantissa, like $14.79 USD, just need to pay for $14 USD.
If the pirce mantissa is more than $0.80 USD, it needs to make the mantissa to $1 USD, like $14.80 USD, it needs to pay for $15 USD.