BUG - Discounts/pricing may not be calculated correctly - needs thorough QA

[jmimico]

In production we sometimes have to provide credits for downtime of service. We do this by editing 'default.py' in the billing api.

Example:
{
'period_start': '2018-05-25',
'period_end': '2019-04-01',
'cpu_price': 0.03,
'volume_price': 0.000067,
'image_price': 0.000067
},
{
'period_start': '2019-04-02',
'period_end': '2019-04-03',
'cpu_price': 0,
'volume_price': 0,
'image_price': 0
},
{
'period_start': '2019-04-04',
'period_end': '2019-11-01',
'cpu_price': 0.03,
'volume_price': 0.000067,
'image_price': 0.000067
},

OR

{
    'period_start': '2019-04-04',
    'period_end': '2019-11-01',
    'cpu_price': 0.03,
    'volume_price': 0.000067,
    'image_price': 0.000067
},
{
    'period_start': '2019-11-01',
    'period_end': '2019-11-02',
    'cpu_price': 0.02,
    'volume_price': 0.000044,
    'image_price': 0.000044
},
{
    'period_start': '2019-11-02',
    'period_end': '2019-11-07',
    'cpu_price': 0.03,
    'volume_price': 0.000067,
    'image_price': 0.000067
},
{
    'period_start': '2019-11-07',
    'period_end': '2019-11-08',
    'cpu_price': 0.015,
    'volume_price': 0.0000335,
    'image_price': 0.0000335
},
{
    'period_start': '2019-11-08',
    'period_end': '2019-11-15',
    'cpu_price': 0.03,
    'volume_price': 0.000067,
    'image_price': 0.000067
},
{
    'period_start': '2019-11-15',
    'period_end': '2019-11-16',
    'cpu_price': 0.015,
    'volume_price': 0.0000335,
    'image_price': 0.0000335
},
{
    'period_start': '2019-11-16',
    'period_end': '2020-12-31',
    'cpu_price': 0.03,
    'volume_price': 0.000067,
    'image_price': 0.000067
}

After reviewing the invoices I am not confident that the discounts are always configured correctly. I am asking that

1. Some QA be performed to the discounts
2. A friendlier way to apply discounts be made