If they don't issue I-20 forms, then you can't study at that university.
The I-20 is vital - you cannot get a student visa without it... and when you have your visa, you cannot even enter the US without showing the I-20 as well (and every time you leave the US while you are studying, you must get the I-20 signed by the university and you present it to US immigration - if you don't have it with you, they will not let you in).
When I first entered the US on my first student visa, I accidentally packed my I-20 in my suitcase... big mistake. I was detained by US immigration for almost 3 hours and very nearly wasn't allowed into the country!
Higher fees are inevitable, I'm afraid, because you will a non-resident student and will be required to pay out-of-state fees (which are a lot higher than the fees for in-state students).
I have the letter of admission and even though UNM does student visas which is nearby I don't want to study there due to high fees and I'm not sure if I meet the requisites to do a course there.
I was a student at UNM in Albuquerque during the 2003/04 academic year and also from January 2008 to August 2008.
I'll be honest - it's not the best university out there, although you might have a relatively good chance of being accepted, since they don't have particularly high entrance requirements. I taught a first-year Environmental Science lab at UNM and many of my students were barely at GCSE-level Science, let alone any higher.
Another option for you if you can't afford high US tuition fees might be to study for a degree in the UK, and do a study abroad year in the US.
I did this with Exeter University - I spent my third year studying at UNM (2003/04). My fees for the year were only £500 (half of the UK fees for the year... now though, it will cost more, since I went to university before the top-up fees were introduced) and my flights there and back were paid by my local LEA!
My student loan covered my living costs (I got £1,000 more that year because I was studying abroad) and the money I earned during the summer gave me some extra spending money
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It was the best year of my life... there was a lot less academic pressure on me than there would have been if I had studied for the entire degree in the US (despite the fact that I studied harder in the US than I did in the UK) and so it was a brilliant experience.