Please keep the change.
TEN-ga — the 'g' is hard (as in 'get'); 'tenga' is the formal imperative of tenere. Stress on the first syllable.
Use this when you want to leave the change as a small tip or gesture of generosity. Tipping is not standard practice in Italian shops but is appreciated for small amounts.
'Tenga' is the formal (lei) imperative of tenere (to keep). 'Pure' adds a sense of 'go ahead' or 'please do'. This is used when the change is a small amount (a few cents) and it would be socially awkward to wait for it. In bars and tobacconists, this is a common polite gesture.
Lasci stare il resto.
Don't worry about the change.
Slightly more casual — 'lasci stare' means 'leave it be'
Tenga i centesimi.
Keep the cents.
Used specifically when the change is just a few cents
Non si preoccupi per il resto.
Don't worry about the change.
The most elaborate version — overly polite for most situations
Leaving small change in Italian shops is an accepted social grace, not a formal tipping practice. In bars it is extremely common to round up or leave small coins on the counter. In a tabaccheria context, the gesture builds the friendly relationship between regular customer and local shopkeeper that is central to Italian neighbourhood life.