So, what really is the difference between the two? Is it something of your level vs the level of the area?
Personally, I'd rule that a "major settlement" for the purpose of item acquisition is a well-connected city of 500,000 people or more - this population can be smaller if the settlement is a major trade nexus (such as a space station dedicated to trade or a city with a significant starport). "Typical settlement" is anything smaller than that, but still a town of significant size.
Mind you, if the PCs are currently in a really small town in the middle of nowhere it would still make sense to introduce an absolute level cap to the items they can purchase... if they need those items in a hurry. But if they have a few days of downtime, and if the town is connected to whatever the local planetary communication networks is, then they can just order the items from abroad and have them delivered there. It's only with the higher-level stuff (i.e. "character level +2") where they will have to make personal appearances so that they can establish themselves as trustworthy customers, and that's why they will need to be in major settlements that have these items on stock.
At least, that's how I would run it - although sufficiently skilled roleplaying can and should circumvent these limits.
To be fair, a Major Settlement shouldn't be something you find around every corner, that would be a Typical Settlement. As such I feel that's appropriate.
And on many planet fast transportation exists. Not including the possibility that the PCs own their own spaceship. Thus, getting to a "major settlement" is largely an inconvenience - it will take maybe a day to get there and another to get back again.