You have one mail delivery service, presumably random-online-shop.com isn’t providing you mail service right? That’s the same thing here.

No, it's not. The power company is analog to Amazon or another webshop. The network company is analog to the postal service.

Your “provider” doesn’t produce the energy.

They actually do produce the energy, the network company doesn't. The power company owns the power plants that actually generate the electricity, or they sub-contract to a company that does (this is a simplification, in reality it's more dynamic than that).

They, like in the packages example you used, are simply handing off the “packages” to your power provider (the mail service) who then administers and provides you with your power (delivers your mail).

You're playing semantic games here.

The network company only transports the energy, it's not theirs. Just like the postal service doesn't own the contents of the packages they deliver. It's briefly in their possession, but it's not their property. And just like with the postal service they are only paid for transport. If I order a €10 bag of cat litter on Amazon, the postal company gets paid €5,95 to deliver it to my door. If I order a €2000 laptop, they also get paid €5,95. If I use 1 watt-hour of energy on a particular day , Enexis gets €1,16 for delivery it. If I use 1000 watt-hours on that day, they also get €1,16.

Let me put it this way: let's say you got a new pair of sunglasses. You meet a friend and he says, "cool shades, where did you get them?". What will you answer?

• "I got them from the postal service"
• "I got them on Amazon"

And again, if all the packages are the same, there’s no difference and it doesn’t matter, why would you sign a contract with any of these companies specifically? Why doesn’t the entire NL just buy from the cheapest company if it’s all the same to you as an end-user of the network?

Because while the actual power is the same, the contracts aren't. There is no "cheapest company", there is maybe a cheapest company for my particular situation. There are many differences between power companies and the products they offer.

• I may want to contract with a company the only produces renewable energy (in fact, I do).
• I may want to have a contract with a guaranteed fixed price for the next, 1, 3, or 5 years. Prices differ for these between companies due to their prediction of how their costs (to either produce or buy) will change in that period. They also change over time. So if my fixed-price contract is up today, company A may be the cheapest option to renew, but if I had to renew 4 months from now, it could be company B.
• I may want a contract with variable pricing, where the price is adjusted based on the price of energy on the European market every 3 months.
• I may want a completely dynamic contract, where the prices vary on an hour-by-hour basis. Even within this there are differences. For examples: some companies will just let you pay the actual market price plus a small markup. This may be risky for the consumer is there is a suddenly a huge spike in price for a few hours (which has happened in the past). Some offer dynamic pricing based different rules, e.g. an average over the last day or so. Which takes away some of the risk for spikes but also means you don't get a lot of savings when the price is suddenly super low or even negative (which has also happened in the past).
• They provide different levels of service. For example: they offer discounts on certain power-saving products, from LED bulbs to heat exchangers.
• If you have solar panels, you can sell the excess power they generate back to the power company. The amount they pay also varies per power company. If you barely use any power at night and you over-produce a lot during the day, you may want to choose a company not based on how much they charge for power, but how much they pay.
• Then there's companies that offer additional stuff. For example: there was at least one that installed solar panels on your roof, and then you'd use the money you'd save in energy usage to pay for the panels in X years.
• Other services that are offered by power companies are things like home inspection to see if there are areas where you cold save power, e.g. by improving insulation.

So it entirely depends on your preferences as a consumer. Do you want a guaranteed price or do you want to take a little bit more risk? How much risk? What kind of service do you want? Just the bare minimum or do you want more?