One of the trickier things about Zapier can be working out how much you're going to need to spend to make your zaps work. Zapier has a variety of pricing plans each with a variety of limits. The two important ones are the number of zaps you can have and the number of task you can use per month.
Keeping it simple, each zap in your account that is switched on counts against your zap quota. If you leave a zap switched off, it doesn't count. This means you don't have to worry about things while you're still building and experimenting.
As for tasks, it's a little more complex. When a zap runs live, each successfully completed step counts as 1 task. The exceptions here is the trigger step which does not count towards you task quota. Nor does a filter step if it does not pass.
Let's look at an example to illustrate.
You have a zap that triggers on a new Shopify order, a filter step to check if the order total is above a certain amount and then an action step that writes the order details to a new row on a Google Sheet. That zap has two steps, excluding the trigger.
If the zap runs, the order total is above the set amount and the details are written to a Google Sheet - then you'll have used two tasks.
If the zap runs but the order total is below the set amount, then the filter will stop the zap. In this case, the zap will not have used any tasks.
With that in mind you can easily calculate how many tasks you might need per month. Say you'll get roughly 50 orders per month above the set total. That works out to 50 * 2 task. You'll be using about 100 tasks per month - so the Starter plan will be more than enough for you.