I'm planning a trip to Mexico and want to stay for the maximum 6 months that they allow me (I am from the UK). After Mexico I am not sure if I will go to South America or come back home (depends on the money situation), so am not planning on buying a return flight. Do I need proof of exit to actually enter Mexico? If so how can I do this cheaply/being able to receive a full refund.
You don't require a proof of exit to enter Mexico. When you enter Mexico, you must complete both customs & immigration forms. The latter is a 2-part form and it asks for an address/hotel where you plan to stay. The INM officer will stamp both parts of your immigration form and give you the smaller section. You must present this form when you exit the country. The INM officer is unlikely to ask you a question - they check & stamp your passport, stamp both parts of your immigration form and give you 1/2 of that form. Enjoy your visit. 7 months ago