My roommate and I have been renting from a private renter for over a year and a half now, and have been exemplary tenants. We sent our landlord July's rent, but because of the holiday it was two days late getting to our landlady. She is charging us a late fee, but the check was postmarked before our due date. We live in Idaho, I do not know if there is a federal standard for this, or if there is any rule or law at all, but any information anybody has would be great! Thanks for your time

What does your lease say? Generally a lease refers to when a payment is received, not sent. Its considered the bill payer's responsibility to post a payment with enough time to get it there by the due date.

Does your lease or rental agreement explain rules governing fees assessed to late payments?

Does the fact of being a exemplary tenant excuse one from not adhering to paying the agreed upon rent on time? No. It's great to hear that you have held up such extraordinary standards while occupying the property but understand this is your landlady's livelihood.

The provided link maybe useful in further explaining this.

Landlord and Tenant Guidelines:

DUE DATES AND LATE FEES:

The lease governs the date on which the rent is due and the consequences for not paying on time or in full. Idaho does not limit the amount a landlord can charge a tenant for a late fee.
Although some landlords will hold post-dated personal checks of their tenants, agreeing to cash
them once the tenant gets paid or receives a sum of money, writing post-dated checks is never a
good idea because the recipient is under no obligation to hold the check. If the check bounces,

the tenant will incur bank fees and possibly suffer negative credit consequences.