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What do you mean by one time only? So if open a channel right now by let's say $100 worth of btc and I buy something for that 100, and then later in the future I want to buy something else for $100 worth of btc using Lightning
from a different merchant I don't have to open a channel? What about from the sane merchant?
Yes, that's how Lightning works.
You single channel is enough as long as it can find a route to the different merchant.
You don't even have to directly open a channel to the merchant that you want to pay, but it's preferred if you're planning to send satoshi to them frequently.
One example if the remote-node is well-connected to other lightning nodes.
If you've opened your channel with the default electrum trampoline nodes or other famous channels,
chances that you wont have to open another channel to lower the chance of failed payment is high.