Each merchant not having the most generic business rule engine for payments possible is creating its own cage

You sold your product, payment by card accepted - perfect! Actually till the next day not even a cent moved towards you.

Perform a normal transaction and then a second, lower amount charge to the same card by mistake. If the customer calls you for a (totally fair) refund, make sure you refund the second charge! If you don't pay attention, you might be getting a chargeback in the end...

Launching a multi country website, it's nice to propose all sorts of currencies. If you have only one legal entity, unless you are overcharged already, all the currencies but your local one will cost you more. Reverse logic: if all currencies cost the same, you are overcharged.

If you use 2 PSPs and you see a difference in the exposure they calculate, it's normal cause each uses different models. If one is too low, watch out it might increase. If your Transaction profile changes, expect both to be revised.

Own your gateway configuration in and out. If you don't, anything might happen and you'll take a long time to fix.

Pay by link does not exjst. It's just ecommerce.

Interesting take on the travel payment side

When deciding how many payment operations to trigger (card validations, incremental), look at your provider contract - you might be increasing the cost of payment beyond expectation

There are thousands of issuers. They all have different systems. They will therefore behave differently even though you think your acquirer is normalising what goes downstream.

Some payment providers propose you to integrate to their "gateway", but actually that's just a plugin bridging you to a real downstream payment gateways. Make sure they provide some additional value, otherwise they are just sales guys.