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I don't know if there is any method to send merit but for sending merit it must be a good quality post. There is no rule to award merit without merit. To send merit, it must be a good quality post only then merit can be given. I've also seen good quality posts get 50 or more merit majors at once. The posts I saw that sent 50 merits or more were very good quality merit posts. Here the post in which he gave merit was not given in full merit post. It is a misuse of merit.
The quality of posts is very subjective - so there is no benchmark that what some people consider low-quality posts is anything but quality to other users. However, to send 50 merit in one transaction there must be a clear reason – apart from the quality of the post, maybe something useful could also be the reason.
I can give you an example:
Look at this one - hugeblack sent examplens as much as 40 merit in one transaction, it's clear why.