What’s the right shipping setup for rural / island US addresses?

This trips up a lot of stores. UPS & FedEx have delivery-area surcharges for rural ZIPs, and they refuse to deliver to many PO boxes / military APO/FPO addresses. USPS covers all of them (including Alaska, Hawaii, Puerto Rico, Guam, US Virgin Islands), often cheaper. We build fallback rules: try UPS Ground first, fall back to USPS Priority for unreachable ZIPs. For Alaska / Hawaii orders we always quote USPS or surface-rate UPS, never expedited. Address validation (USPS API or Smarty Streets) catches typos before the label prints — saves real money.