Separate contributors from names on the card
A person may be happy to sign a group card without agreeing to fund a purchase. Ask contributors individually what they are comfortable spending. The person buying needs to know the total available; other relatives do not need a list showing which adult offered which amount.
Build the gift around the amount offered
For a $40 total, an item costing $36 plus $3 wrapping fits. An item costing $39 plus $6 delivery does not. Before ordering the second item, choose a lower-cost delivery option, a different gift, or an explicitly agreed larger contribution. Do not send a request for $22.50 each after promising a $20 limit.
Handle an offer that arrives late
When somebody offers to contribute after the purchase, confirm whether the original buyers want reimbursement or prefer to keep it as their gift. Keep the card message separate from the payment details so nobody has to explain their circumstances to the recipient.
A late contributor might choose to bring flowers or pay for postage instead. Tell the original buyers what changed and write down the final shares. A change to the payment arrangement need not change the card or the way you present the gift to its recipient.