BitPay invoices can be paid from an up-to-date Bitcoin or Bitcoin Cash wallet that supports payment protocol. A list can be found here.
The bitcoin wallet you should use for paying a BitPay invoice depends on the operating system and the platform (mobile, desktop) that you're planning on making the payment from. Please note that using a bitcoin wallet that auto-populates the bitcoin address, payment amount and dynamically calculates the miner fee to include in the transaction greatly increases the likelihood of successful payment and timely confirmation.
BitPay's wallet is our first choice and recommendation. It is an easy, friendly, and full-featured wallet available on all desktop and mobile platforms.
If you have already downloaded and setup a Payment-Protocol compatible wallet, then you will be interested in looking at the following article which explains how to quickly and safely pay a BitPay invoice https://bitpay.com/pay-with-bitcoin.