The delivery estimate you see at checkout has two parts: how long the seller needs to prepare your order (handling time), plus how long the carrier takes to deliver it (transit time).
Handling Time
Each seller sets their own handling time — typically 1 to 3 business days — and a daily order cutoff time. Orders placed before a seller's cutoff are counted from that day; orders placed after the cutoff, or on a day the seller doesn't process orders, are counted from their next processing day.
Transit Time
Once your order ships, the remaining time is carrier transit — how long delivery takes once the package is picked up. This varies by shipping method (standard vs. express) and destination.
The estimate you see at checkout already combines both, so it reflects the full time from placing your order to it arriving at your door, not just the shipping time.