I stand by my statement that it's not Mustachian. A core component of being Mustachian is reducing consumption. This of course has financial benefits, but another big motivator for reducing consumption is to reduce the impact we have on our planet. You might be able to justify Prime financially, but if you look back at most of my posts in this topic, you'll probably note that consumption is my biggest complaint, not necessarily the finances of it.
You've given some good examples in other threads of ways that you have Mustachian habits like buying used items instead of new, doing your own repairs to extend the life of your possessions, and growing a garden. Those are very Mustachian things to do because they save money andreduce consumption at the same time. Paying monthly money that you don't need to in order to have "free delivery" of small trinkets so that you can skip a trip into town, or avoid some basic planning and combining trips is not Mustachian. A big reason that I say that, is the wasteful consumption it causes. You might not feel the environmental impact, but the impact is still being made. It's there in the unnecessary trips by inefficient vehicles. It's there in the extra packaging. It's there in any impulse purchases that might not otherwise be made. That's why I say it's not Mustachian. You're paying extra money, and increasing the environmental impact of your consumption. It's the exact opposite of your Mustachian habits.
