Rockpoint, a sports equipment retailer in Prague, said on Thursday it would give a 20% discount to anyone who turned in unused ballots for the communists and the leftist ruling party, the Social Democrats.
"Do not vote for the red coalition. Vote for a 20% discount," the firm said on its website.
A similar pledge to Rockpoint's appeared at an antique store in Prague.
Mowshe, a graphic studio, is running a campaign urging voters to send in as many communist ballots as possible, and said it had received about 2,500 so far, including 12 collected by one family.
Czech voters get ballot papers by post for each party.
Michal Gregorini, of Mowshe, said the winners would receive prizes such as anti-communist T-shirts and a subscription to the magazine Respekt, an independent weekly.
The communists have had no share of power since their Soviet-backed administration ended in 1989, but the Social Democrats say that after the election they may try to form a minority cabinet dependent on communist support in some key votes.
The Social Democrats lag slightly behind the right-wing opposition Civic Democrats in opinion polls.
No party stands a chance of winning an outright majority, so any winner will need partners to form a coalition government.
Unlike their counterparts in several other former communist countries, the Czech communists did not change their name and have not made a clean break with their past.
Final opinion polls on Thursday put the rightist opposition Civic Democrats slightly ahead of the ruling Social Democrats, with only three others - the centrist Christian Democrats, the communists and the Green party - seen passing the 5% threshold to gain entrance to the lower house.
Czechs will cast their ballots on Friday and Saturday.