The plugin is always in one of two modes, and knowing which one you are in saves a great deal of confusion.
What test mode does
Everything behaves exactly as it will in real life. The checkout appears. The payment screen opens. The plugin processes the result. Emails go out. The package activates. The transaction appears in your admin.
The only difference is that no money moves.
Mollie gives you a test payment screen where you choose the outcome yourself: paid, failed, expired, cancelled. That is genuinely useful, because it lets you see what your customer experiences when something goes wrong, not just when everything goes right.
What live mode does
Real payments, real money, real customers.
Switching between them
Open the API keys tab and change Active mode.
Two things are worth knowing.
Each mode needs its own key. Test mode uses your test key, live mode uses your live key. If you switch to live and you have not filled in a live key, nothing can be paid at all.
You can switch back. If you want to test something after going live, switch to test mode, do your testing, and switch back. Your live data is not affected.
Test data does not pollute your real figures
Test transactions are marked as such, and the plugin can clear them out for you.
On the Analytics tab there is an option to reset your test data. It removes the test transactions and leaves your live data completely untouched.
Do this once, on the day you go live, so your reporting starts clean.
When to use test mode
Always, before you go live. Non-negotiable. Do at least one full test payment from your customer’s point of view.
After any change to your packages or payment methods. If you change what you sell or how people can pay for it, test it.
Whenever something looks wrong. Test mode is free, and it is faster than guessing.
The mistake to avoid
Do not leave the plugin in test mode after you launch.
It is an easy thing to do: you finish testing on a Friday, you tell everyone you are live, and you forget to flip the switch. Your customers then go through a checkout that takes no money, and you have a very strange Monday.
The TEST badge at the top of the plugin is there precisely to stop this happening. If you can see it, you are not taking money.