The Matukio works with sending alerts emails to persons registered at the event?
What do you mean with alerts email?
The registered person and organizer get a booking confirmation and some other emails when events change (all customizable with settings and templates).
Where can I find documentation that can help the developer to create a payment plugin?
You can find that at the techjoomla payment API page and at github.
techjoomla.com/common-payment-api
It is possible to send me a copy of a payment plugin that will be useful in the development of another plugin?
You can use any plugin provided by techjoomla (i always suggest paypal, but any other will work too) for a starting point, but in your case i would use an existing pagseuro plugin (as i said i am still not 100% sure what's going on with it..) .
github.com/techjoomla/joomla-payments/tree/master/code/plugins
Kind regards,
Yves