There are many ways you can do this (I've purchased over 8 of these for many of my co-workers that all came to the liking of WindowsPhone after I showed was them features of mine),I'm from Caribbean also,most of the merchants sell you AT&T Go Phone with a paper with unlock code to enter into your phone once you unbox ,I had issue once purchasing from CrazyLowPrices Merchant whereby the paper included in packaging had the phone's IMEI # as the lock code,contacted seller,wanted to do replacement of phone once phone is returned but i live in Caribbean so that would be costly and lengthy also,I then used
Unlock Codes & Network Cellphone Unlocking | CellUnlocker.net to purchase unlock code ,contacted back CrazyLowPrices ,they refunded me the cost of unlocking the phone which was about 30 bucks and took 3 days to email me code. I then didn't want to go through that anymore so I contacted all merchants that are selling the Lumia 640 unlocked on Amazon and apparently EmilyStore merchant is only one that ships phone unlocks without you having to put in codes(less hiccups of any sort) and offer Prime Shipping too to get fast if you have Prime subscription.EmilyStore cost little more than others ,$110 @ my time of purchases.What you can also do is purchase during Best Buy Black Friday deal whereby BestBuy selling Lumia 640 for $30,AT&T unlock for another $30,get screen protector,micro SD and case,that should bring you to around 110 & you'll get phone with needed accessories,at least that's what I'm doing for some other workmates,so far converted about 10 workmates and will have about 4 more with Black Friday coming up,even showed some The October Devices Presentation and some planning on getting Lumia 950 also.