Hi! I have a question about unlocking the phone for Russia when buying in the USA, If I buy an iPhone in the US, how do I unlock it there and how much does it cost?

iPhones sold in the USA might have different hardware installed, when it comes to using LTE and other mobile data networks, since iPhones are supposed to be used in the country of purchase.

Also, the warranty and service options can be restricted to this country of purchase by Apple. In case of service, you will have to send or bring it back to the USA, since Apple Support does not ship internationally.

If these things do not matter for you, you'll have to buy an unlocked iPhone from one of the Authorized Apple Resellers or from an Apple Store. Other iPhones might be locked to a carrier, that has to unlock it for you:

How to unlock your iPhone for use with a different carrier - Apple Support

Here's some information about that:

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Tax rate

In accordance with state and local law, your online purchases from Apple will be taxed using the applicable sales tax or seller's use tax rate for your shipping address, or the Apple Store address where the item is picked up. 

The tax listed during checkout is only an estimate. In California and Rhode Island, sales tax is collected on the unbundled price of the iPhone.

Your invoice will reflect the final total tax, which includes state and local taxes, as well as any applicable rebates or fees.

Tax-exempt orders

To place an order on behalf of a tax-exempt organization or individual, please call an Apple Store Specialist at 1‑800‑MY‑APPLE. You will need to provide proof of state tax-exempt status for the state where the product is being shipped. The organization or individual name on the order must exactly match the state tax-exempt certificate. Federal tax exemption certificates are not applicable.

Note, for corporate tax-exempt purchases, only corporate-issued credit cards for the applicable tax-exempt organization will be accepted as a method of payment (i.e., no cash, personal check, or personal credit cards will be accepted).

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copied from: Payment, Financing, Refunds and VAT - Shopping Help - Apple