It is often underestimated that the accessories cost good money. In hardware stores, mostly junk is sold that hardly meets the requirements. Bursting pipes and so on are no joke and can quickly ruin you. One should keep in mind that the pipes etc. are later permanently! under pressure, even during vacation or at night.
You also cannot compare the online prices with the plumbing prices. The internet prices are so low also because theoretically no one is allowed to install them. The water supplier’s regulations, for example, basically always stipulate that work on water pipes may only be carried out by professional companies. Of course, no one really adheres to this and normally everything goes well. In the worst case, your contamination due to improper installation could get into the neighbor’s water during a pressure drop at the waterworks and kill him after his serious operation and his weakened body. Then you would still have to find out that it was due to the water and then that it was because of you. But then ^^
a lot of disasters but possible. You’re more likely to win the lottery twice with 6 correct numbers.
Conclusion: buy the stuff online, but also all connection accessories only in the "original" from specialist dealers and no hardware store junk. At least the parts that are under pressure...