The Buyer undertakes to purchase goods whose cost is less than....

Это правильно??

animate: 'The woman WHOSE glasses are too big'
inanimate: 'The house, THE windows OF WHICH are too big'

Most people find the construction 'the ...... of which' too wordy, so as no-one's yet invented a word such as 'whatse', we cheat by using the animate 'whose' instead:

'the house WHOSE windows are too big'.

That's the only reason. If it helps, lots of English people think it sounds slightly odd too. Perhaps I should invent and patent the word 'whatse' and make myself very rich...?

The BBC uses it:
* "Quentin Willson drives the car whose avant-garde lines took France by storm in 1955"
* "a house whose garden backed onto the one I was in"

and Collins' English Dictionary defines it as:
"1. a. of whom? belonging to whom? used in direct and indirect questions ⇒ I told him whose fault it was, ⇒ whose car is this?
b. (as pronoun) ⇒ whose is that?
2. of whom; belonging to whom; of which; belonging to which: used as a relative pronoun ⇒ a house whose windows are broken."

If it's good enough for Collins and the BBC, it's good enough for me. It may not sound ideal to linguists/grammarians like you and me, but everyone else uses it -- because there's no other single word they can use.

Роман, на сей раз все-таки сдержусь в выражениях и буду учтив. но строг, как всегда. нельзя сказать "the house that has too big windows". это неграмотно. я понимаю, Вы ненарочно. и, к Вашей вести, я заметил, что Вы стали реже претендовать на "нативность" и истину в последней инстанции в том, что касается США и тамошнего варианта английского. продолжайте в том же духе. the sky is the limit.