This is part of their Love it or we will refund your money promotion:
http://activia.us.com/promotions

a) The original store-identified cash register receipt dated between 12/22/11 – 3/15/12 with the
purchase price circled, and
b) Name, address and a brief description of why you didn’t see the benefits of ACTIVIA.

It's not perfectly clear, but the use of the singular "receipt" suggests that it has to be on a SINGLE receipt.
"To be reimbursed for the cost of ACTIVIA yogurt, purchased between 12/22/11 – 3/15/12, follow the directions below. Requests must be postmarked by 3/31/12 and received no later than 4/15/12.
To receive a check for up to $36.00* please complete this original redemption form and
mail it with the original store-identified cash register receipt(s) dated between 12/22/11 – 3/15/12 with the purchase price circled"

" Actual refund check will be your ACTIVIA purchase amount up to a maximum
refund of $36.00, not including sales tax or the amount of any coupon or discount
received at the time of purchase.
Terms and Conditions: Limit to one (1) submission per family, household or address for a given game period. Specific store receipt may be used for only (1) participation per household per each specific Dannon promotion. Only one request per stamped envelope. Offer expires 3/15/12. VOID in LA, Puerto Rico, US Virgin Islands and where prohibited or restricted by law. Valid in 50 U.S., and District of Columbia. Allow 6 to 8 weeks for mailing of check. See www.activia.us.com for official rules "

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