I sent less than required because of the network fee
Every blockchain transfer carries a network fee. If you sent exactly the invoice amount and your wallet or exchange took the fee out of it, less than the invoice amount reached the payment address — and the payment wasn't credited in full.
What to do
1) Find your transaction hash (TXID) and check in a block explorer how much arrived. 2) Contact the store with your order number, the TXID and the missing amount. The store decides whether to accept a partial payment, ask you to top up or refund you; some stores accept small underpayments automatically — it depends on their settings. 3) If the payment page itself offered to let you pay the difference, top up there. Otherwise, top up only via a new invoice or the store's instructions: the address from the old invoice may no longer work.
How to avoid it
if your wallet or exchange allows it, enter the amount to be received rather than the amount to send, and check how much will arrive after the network fee before you send.