There's a fair bit of 'receipt'-looking paperwork:
Modern small transactions combine most of these into one bit of paper.
The 'three receipts' were likely two receipts and one bill of sale. The government is interested in the paid-down invoice.
As bike purchasers the moment of the 'bill of sale' is important when ordering a bike. If you fully pay for a bike which is to be provided later then ensure you get a bill of sale (eg, listing the serial number of the bike). Then if the bike shop enters receivership the bike is yours. If the bike shop doesn't offer that, then don't pay any substantial sum in advance.