Thanks to @suneelmunj for raising the issue in this week's Pakwheels Weekly. I think this post will become good ground to put up a fight.
Also I got a chance to study consumer laws and Sales Tax Law by virtue of this group.20217161373354644SalesTaxAct,1990updatedupto30.06.2021.pdf (2.1 MB)
As per the law, Provisional Booking Order is not the invoice. They issue PBO because as per Sales tax ordinance the tax is charged on date of supply of goods and not on date of booking and the manufacturers don't want to be penalized on delay of delivery of goods due to issuance of invoice. Now when they have issued invoice they are illegally overcharging the tax.
Upon complaints to multiple fora, it has been found out that SECP and Ministry of Commerce endorsed the point of view of consumers in this group and have directed FBR to take up the matter as per prevailing law.