Once Turkmenistan letter of invitation (LOI) has been issued/ approved you can get the actual transit visa without any difficulties. If you are entering Turkmenistan through one of the land borders you don't actually need to do anything in advance. All you need to do is rock up at the border with some cash in USD (between $60 and $100 depending on nationality), copy of this LOI and your passport. The visa will be stamped into your passport there and then and you'll be able to stay in Turkmenistan for up to 5 days from the date you enter the country.
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