Generally, a one time thing is considered a gift. For the donor this is obviously not a deductible expense, except for some specific cases (for example promotional gifts under $25 to vendors can be deducted, if you're a business, or charitable contributions to a recognized charity).
However, if this is a regular practice - that would not be considered as a gift, but rather as a tax fraud, a criminal offense.
Being attentive I would like to make a little gift or give some little
(<100$) amount of money (cash/wire/online) for that
Why? Generally, gift is exempt from income if no services were provided and the gift was made in good faith. In the situation you describe this doesn't hold. When the gift is exempt from income to the receiver - the donor pays the tax (in this case, below exemption the tax is zero). If the gift is not exempt from income to the receiver - it is no longer a gift and the receiver is paying income taxes, not the donor.
The situation you describe is a classic tax evasion scheme. If someone does it consistently and regularly (as a receiver, donor, or both) - he would likely end up in jail.