Assume I'm a buyer and I want to guarantee a seller that I will not charge back my Paypal or GoogleCheckout payment. Is there any binding mechanism to do it in those systems? Or may be there is a similar online payment system that has this feature?
Not really. You can promise, but that would not prevent you from actually doing it. The seller can then claim "he promised" to PayPal, but PayPal usually don't care about seller claims, and I assume Google wouldn't either. These companies only care about their bottom line, and do not take any risks, so in case of chargebacks - sellers are usually screwed.
The seller can sue you in court, though, as your promise is in fact a contract, but if you're in a different country it may be a hard thing to do. That is why many sellers do not ship internationally on eBay, for example.
An irreversible payment option would be a wire transfer through your bank or companies like Western Union or MoneyGram. It is more expensive, true. But you do get something in return, as you can see.
This is not entirely a serious suggestion, as in, I can't really recommend it, but it immediately came to mind as meeting your request for an immediate online payment system without chargebacks:
Bitcoin inherently has no chargebacks.
Of course, both parties have to be willing to use Bitcoin, and exchange the money for more conventional currency at both ends with accompanying transaction fees. And the recipient has to wait a few minutes for certainty the transaction is confirmed. And there is the legal uncertainty, and the varying exchange rates, and so on. But it does meet your stated criteria.
Depending on where you are - in the UK at least, you can do a BACS transfer (direct bank transfer) if the seller provides his/her bank account sort code and account number. In the UK this is free and if both banks support "Faster Payments" it is more or less immediate.
The same system may exist elsewhere, I don't know.
Here's PayPal's policy:
If you meet certain conditions and you are a US account holder, you will be covered in the event of an unauthorized payment, an item not received claim, or a chargeback. Check out our Seller Protection Guide or FAQs for more information.
Here's a specific item in the FAQ:
Will Seller Protection give me access to funds if a claim or a chargeback occurs?
PayPal will place a temporary hold on the transaction amount while we make sure the transaction is eligible for coverage. Once we validate your shipping documentation, if the transaction is covered under Seller Protection, we will lift the temporary hold and restore your access to the funds. If it is not covered, we will return the funds to the buyer's account and you may be responsible for PayPal's chargeback fee, if applicable.
The odd thing is that the official PayPal FAQ on this doesn't seem to define what you need to do to be "eligible for coverage"
From my experience as a seller, PayPal has a dispute system that kicks in on chargeback. If you can provide reasonable documentation as specified in the FAQ, the seller will receive the funds.
Google Checkout seems to have a similar policy (source):
We'll evaluate the chargebacks you receive and, whenever possible, fight them on your behalf. Your Checkout account won't be debited for the chargeback if Google can dispute it successfully with the supporting documentation we’ll ask you to provide.
It seems that the Google Checkout chargeback policy applies only for goods, not services, as documented here. Seller beware!