My mom couldn't donate regularly because we were dirt poor, but when she made some big money on a job and FINALLY had a few pennies, instead of buying me new shoes I will never forget she would put 1/2 the money in the box to make up for all the times she couldn't donate. Sometimes that would be $500 at a time. For us living in a trailer park, that was HUGE money.
The contribution/donation arragement also made it harder on Witnesses if they didn't get a substantial enough donation, some would feel obligated to pay for literature that they placed if the householder didn't.