Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Why do you think you can trust online "reviews"?
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I looked at the Plaza site. The Home and Student page is very good. Just a couple of grammatical errors that can be written off to ESL. No apparent factual errors, which are the usual clue of scams.
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The one thing I could not find on the site was any explanation of how they can sell so cheap. The only "legitimate" way I can think of is buying stock from bankrupt companies. The only other way I can think of "it fell off the back of the truck" (aka stolen goods).
How can they sell any legitimate Office 2019 license for $30, or even the "original" $90 price. I could maybe believe $30 for Office 2010 because it was a "going out of business" (end of life) "sale", but not for 2019.
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It's a balancing act. $30 is roughly 1/5 the retail price of H&S. 2019 has a little less than 5 years to EOL, so if you can get 1 year of use before MS blocks it you got your money worth, any extra time is "bonus". Maybe you get lucky and you get to use it to 2019 EOL ... Maybe you don't. Can you afford to gamble the money? For me that is roughly 3 weeks of lottery tickets. Or roughly equivalent to a case of beer, or 4 or 5 packs of cigarettes ...
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But, at this price, even though they say Home and Student, you may be "pleasantly surprised" they "accidentally" gave you Pro Plus ... a variation on 'bait and switch' advertising.