SEO agency offers to work on my ecommerce on-page SEO optimization for one payment of 500€. Is it worth it?

Good evening,

I am currently in talks with one SEO agency and I am planning to hire them for general SEO work - website audit and error fixes, keyword audit for certain topic, create content plan and strategy, optimize blog posts and create new ones, get 23 various types of backlinks a month.

However, they really strongly recommend to purchase On-Page SEO optimization + loading speed optimization for total of 500€, one time payment.

My question would be: is it really necessary to spend money on On-Page optimization before doing any other SEO related work? Are they asking too much money for it?

Thank You.