Billing
When your card is charged, and how daily proration and graduated tiers compute your invoice
Zernio is usage-based: rates live on the Pricing page. This page explains the mechanics behind the invoice, when your card is charged, how daily proration works, and how the graduated tiers apply to your monthly total.
When your card is charged
We charge your card automatically as you accrue usage. The first time your accrued usage reaches a small fraud-protection threshold (starting at $10), your card is charged for that amount and the threshold doubles for next time. Established customers see fewer, larger charges; brand-new paying customers see one quick small charge to validate the card.
Invoicing is powered by Metronome on top of Stripe. Each invoice is itemized: you see exactly which accounts, phone numbers, messages, calls, and X API operations you're paying for.
Daily proration
Accounts are metered per day, not per month-snapshot. Every day, our system records which accounts are connected and reports them to the billing engine, one event per active account per day. The per-day mechanism means you only pay for the days each account was actually active.
For a single account in a single tier, the cost works out to:
account_cost = tier_rate × (days_active / days_in_month)So a $6/month account connected for 15 days costs $3 that month. If you connect on day 1 and disconnect on day 15, you're billed for the half you used.
Graduated tiers on the total
Because pricing is graduated ($6 / $3 / $1), the rate isn't fixed per-account. It depends on your total billable units for the month, computed as:
billable_units = (sum of all account-days in the month) ÷ days_in_monthThat total is then run through the graduated tier ladder (first 10 units at $6, next 90 at $3, next 1,900 at $1). You never get charged tier-1 prices for units that fall into tier 2 or 3: partial usage in higher tiers gets the lower rate.
Mental model: at the end of the month we add up every "account-day" (one per account per day connected), divide by 30, and run the result through the rate ladder. So 100 accounts × 3 days costs the same as 10 accounts × 30 days. The graduated rates apply to the total, not to individual accounts.
The calculator on the pricing page reflects this exact math: account-days → divide by 30 → graduated tiers → minus $12 free-tier credit.
Worked example
A customer has 10 accounts connected for the full month. In week 2 they connect an 11th account, then disconnect it in week 3. In week 4 they connect a 12th, different account, for the rest of the month.
Step 1 — count account-days:
| Account group | Days active | Account-days |
|---|---|---|
| 10 baseline accounts | 30 days each | 300 |
| 1 account (week 2 → 3) | 7 days | 7 |
| 1 account (week 4 → end) | 7 days | 7 |
| Total | 314 account-days |
Step 2 — convert to billable units:
314 ÷ 30 = 10.467 billable unitsStep 3 — apply graduated tiers:
| Tier | Units in this tier | Rate | Cost |
|---|---|---|---|
| Tier 1 (1-10) | 10 | $6.00 | $60.00 |
| Tier 2 (11-100) | 0.467 | $3.00 | $1.40 |
| Total gross | $61.40 |
Step 4 — apply the free-tier credit:
| Item | Amount |
|---|---|
| Total gross | $61.40 |
| Free-tier credit (flat per month) | −$12.00 |
| Net invoice | $49.40 |
The total bill is not "11 accounts" or "12 accounts". It's the sum of all account-days converted into billable units, then run through the graduated rates. The 11th and 12th accounts (which only existed for partial weeks) push the total just past the tier-1 boundary, so their fractional contribution is priced at tier 2's $3 rate, not tier 1's $6.
Common questions
- What window does Zernio measure activity in? Calendar month — account-days accumulate from the 1st to the last day of each month, then flush into a new period.
- If I disconnect and reconnect the same account, does it count twice? Each day it's active counts. If it's active for 10 days, disconnected for 5, then reconnected for 10 more (same calendar month), that's 20 account-days for that account.
- What if I connect a brand-new account mid-month? Charged from the day it's connected onward, prorated through end-of-month.
- Which accounts pay tier-1 vs tier-2 rates? The graduated rate isn't tied to individual accounts — it applies to the total billable units. The first 10 units of usage are at $6, units 11-100 at $3, and so on. So if your monthly total works out to 12.5 billable units, the first 10 are billed at $6 and the remaining 2.5 at $3.
- What about the free-tier credit? Each calendar month grants $12 of credit (covers the first 2 accounts at the $6 tier). Credit applies to the gross monthly total — it's a flat per-month grant, not per-account.