The one-cent difference doesn't come from a bad calculation; it comes from rounding too early. If each tax is rounded on its own and then added up, a 1,200.00 MXN rate can end up charged as 1,199.99 MXN. Containn follows three rules so that doesn't happen.
1. The net is stored to six decimals
For a rate, the system stores its net amount, before taxes, to six decimals. For example, a hotel that publishes a room at 1,200.00 MXN including IVA (Mexico's value-added tax, 16%) and ISH (the state lodging tax, 3% in this example) stores a net of 1,008.403361 MXN per night. Truncated to 1,008.40 MXN, a three-night stay would come to 3,599.99 MXN instead of 3,600.00 MXN.
You never see those decimals: everything on screen shows two. When you enter a tax-inclusive price, the system chooses the net so the night and the reservation total land exactly on what you published.
2. Round once, at the end
Each tax is calculated on its base with full precision, and only the total being charged is rounded. The same goes for each night the night audit posts: the night's total tax is rounded once, the night's amount comes from that, and the cent separating the individual roundings is assigned to the largest tax so the folio's columns add up to the line. With the example above:
| Item | Amount |
|---|---|
| Net, to two decimals | 1,008.40 MXN |
| IVA (16%) | 161.35 MXN |
| ISH (3%) | 30.25 MXN |
| Night total | 1,200.00 MXN |
A service charge, when there is one, goes into the same rounding. A fixed-amount tax is charged once per reservation, not per night, and isn't part of the rate.
3. In the books, everything balances to the cent
The póliza (the Mexican term for a journal entry) records each line in cents and is not saved if debits and credits differ, even by one cent. There's no plug account to absorb the difference: an imbalance is shown, not hidden.
If a line's per-tax breakdown adds up exactly to its total tax, each tax goes to its own account. If it doesn't add up exactly, the full tax goes to the general taxes-collected account. It is never prorated.
Two more cases
- Extras the front desk adds to a room stay in cents, exactly as entered; the room absorbs the remainder, so the parts always add up to the total.
- When a stay's total is split by night, each night keeps its decimals and the last one takes whatever is left, so the sum is exactly the total.
If a rate, a folio or a póliza shows a one-cent difference against what was published, don't adjust it by hand: report it. These rules exist so it doesn't appear, and if it does, it needs to be fixed where it started.