Thanks for all of your replies guys! very appreciated thankfully,
Just to clarify one point that I checked the eVisa option already, this one offered for a list of around 80 countries as per the website displayed, the passport is not among this list,
Also, I searched this morning and emailed some travel agencies explaining the purpose of this letter, they trying to offer full package not just letter, package with hotels booking,
I repeated focusing only to the letter, they said only can offer with booking,,, imagine it..!!
As I remember this kind of invite letter, I made it in late 2018 from Russia because it was required by the Russian embassy here in BKK, it costs me around 16 - 18$ that time, and received it in the same hour, everything done online, super easy and fast, done from a hotel and no any obligation to stay with them or not, separate service,
So, this time I learned it was requested by vietnam embassy just for visiting visa to travel for a week or 2 to buy some items, this letter before the covid19 was not required,
I wonder what this letter will help for? or just indirect way to sell packages?
ANyway, I won't and will never go for these packages or any kind of taking advantages like this, as no any necessary purpose to travel to there, just incase this letter obtained within reasonable cost,
if anyone done this and obtained this kind of inviting letter, please share your comment with many thanks in advance!