The hardest thing about buying SEO is that you cannot judge it for months, and by then you have
Shopify SEO is mostly structural — indexation, product data, page weight — and an agency that only offers content is offering half.
Reported rankings and traffic are easy to present favourably, so agree in advance on metrics that are hard to game.
The first month should be diagnosis, and a proposal that skips straight to deliverables has not looked at your store.
Ask what they would tell you not to do; an agency with no such answer is selling scope rather than judgement.
It varies enormously by scope and market, and the more useful question is what the engagement covers. An audit is a defined piece of work with a deliverable.
The full guide has the formats, the deadlines and the worked examples.