driving fines

To the best of my knowledge, fines don't change between municipalities.

Non-residents have to pay 'on the spot'; residents have 20 days to pay.

Speeding fines increase with the amount over the limit, but you would expect around €100-€140, plus points on your licence, again depending on the amount over the limit.

Failure to wear a searbelt is a €200 fine, reduced to €100 if paid promptly.

You need to carry your licence, car registration, insurance, proof of address in the car.

As with the UK, you have the right to contest a fine, however, in my personal opinion, your story versus that of 2 police officers or a speeding camera is unlikely to succeed with some substantial supporting evidence.

Hope this helps!

Do You Get a Discount For Early Payment?

All fines must be paid within 60 days. If you pay the fine within 20 days of receipt, you will usually be given a discount of around 50%.

If you do not receive a fine within three months of a minor offence the fine is revoked. For more serious offences a fine is cancelled after six months.

If you have a digital certificate you can register with the DGT 
so they notify you if a fine has been imposed. This makes it easier to grt it paid within the 20 days. The DGT website gives a telephone number which you may use to pay, also with English option.

I want to know if I can challenge a fine, such as 200/100 Euros for not wearing seat-belt. It seems excessive for forgetting when driving from one shop to another.

Yes of course you can challenge the fine for not wearing your seatbelt.

However the chances of your challenge succeeding are virtually nil.

You were either wearing it or not unless you have a medical exemption you are stuffed.

Or you forget.

No excuse under the law.

I can guarantee that you will lose any appeal. Sorry but they will not accept forgetting as a reason to cancel the fine.

Is this opinion from personal experience or only a guess ?

Not a guess. I have a friend who is a lawyer in Spain and he says you would be wasting your time.