Eduram - one thing to consider is that visa offices abroad cannot process online payments (yet) so if you pay the RPRF online, you'll need to wait for CPC-M to process the online payment and reflect the RPRF as paid in your file before Mexico can issue your PPR. Therefore, I usually suggest this only if folks are not yet very close to having the CoPR issued.
In contrast, most local visa offices (including Mexico) can process RPRF payments in local currency themselves and thus if you mail it in with the other documents they requested they can process it and reflect the payment in your record immediately. The link I gave you the other day has the instructions as the specifics vary between banks re number of bank officer signatures.