Since Uquid uses bitpay's invoice system, the transaction expires within 15 minutes if there is no (single) confirmation for the transaction with the amount mentioned in the invoice.
That is incorrect.
Bitpay invoices
do not expire within 15 minutes if there is no confirmation. You have 15 minutes to
broadcast the transaction on the network before the invoice expires.
I have purchased several items from several different merchants (eGifter, Steam etc) who all use Bitpay as their payment processor. As long as you broadcast the transaction (and make sure the amount you send is correct) within the 15 minutes, Bitpay detects the "unconfirmed transaction" and just marks it as pending. They were even nice enough to give me warnings that my fee was too low and the transaction make take a while to complete.
Later, in some cases several hours later, when my transactions finally got a confirmation, it was marked as such by Bitpay... and the merchants processed my orders.
Given that the block time is an "average" of 10 minutes... there are instances where the time between blocks has been over 15 minutes... how are you supposed to get a confirmation (even assuming you pay the best fee and get included in the next block) if there are no blocks mined in the 15 minute window that Bitpay provide? Bitpay understand this and that is why it isn't based on confirmations. You just need to send the transaction. Check out the block times from the last 10 blocks: