How do I prevent a customer from being late-fee-eligible?

Open the customer's profile and set the Late fee rate to 0. New invoices for this customer will then use a 0% daily late fee, even if your business default is non-zero. Existing invoices keep whatever rate they were issued with.

Leaving the field blank falls back to your business-wide default — only an explicit 0 overrides it to "no late fee for this customer". For the full per-customer override list, see Setting defaults for individual customers. To turn off late fees globally instead, see the late-fee section of Customising invoices and estimates.