Pro Site Quota Upgrade Purchasing

Can we add a feature that will allow the users to purchase more space for their site and not be limited as to how much they can buy? That way they don't have to upgrade their plans since they pay it once yearly and I won't need to make a probated refund and make a ton of different plans with different quotas. This way its just the 1 plan I have with the bulk pricing plans. Also could you make it so that way you can get bulk pricing for quota upgrades too? Thanks, appreciate it guys. - Brandon