Can I work with you on just one service or do I need to take the full package?

The real constraint is not commercial, it is sequential. Some work depends on decisions made elsewhere, and buying it out of order produces something that has to be redone. A website built before anybody has settled who the customer is and what they are being told will describe the business you assumed you had rather than the one you turn out to have. Advertising bought before conversion tracking exists spends money you cannot measure. Search work started before the site can convert sends qualified people to a page that does not ask them for anything. In each case the single service is not the problem. The missing prerequisite is.

Some things genuinely stand alone and are worth buying that way. Setting up analytics and conversion tracking properly. Claiming and completing your business profile. Fixing a site that is slow or unusable on a phone. An accessibility pass. Documenting how your existing tools connect so you are not the only person who understands them. None of those require anything else to be true first, and each one produces value on its own.

If you ask for a single thing that will not work in isolation, I will tell you what has to be true first and what it costs, and then it is your decision. Sometimes the prerequisite is a conversation rather than a project, in which case it is included and we move on. Sometimes the honest answer is that the thing you asked for is not what will help, and the more useful work is smaller and less exciting. That happens often enough to be worth mentioning in advance so it does not feel like a bait and switch when it arrives.

What I will not do is bundle something in because the invoice looks better with it. A business in its first year has limited money and every dollar spent on something premature is a dollar unavailable for something necessary. If two of the three things you asked about can wait six months, the recommendation will say so, and the engagement will be smaller than the one you came in expecting.

If a single piece is all you want, say so plainly at the start rather than expecting the conversation to arrive there. Being told directly that you want a website and no advice about anything else is useful information and it is entirely reasonable. It shapes the engagement into something narrower and cheaper, which is a better outcome than a broad conversation neither party wanted.