What is the "Send Max" feature in the BitPay Wallet?

Sending a bitcoin transaction is like sending money through the postal service. To mail a single $100 bill, the shipping cost is inexpensive. However, if you're trying to send 10,000 pennies, shipment for the same amount of money will require a larger package and be more expensive.

Similarly, bitcoin fees are sensitive to the size of the transaction. Multiple transaction inputs will make your transaction bigger, regardless of the value of those inputs.

Using the BitPay Wallet, you can send your funds using the Send Max feature. This will combine your wallet’s inputs to make the fees less expensive. The Send Max feature determines the amount that can be economically sent from all of your inputs. 

To use Send Max:

• Open the BitPay App.
• Go to the settings tab by clicking the gear icon on the tabs bar (at the bottom).
• Select the wallet that has the funds you want to move.
• Click Send.
• Select the destination wallet. You can send the funds to the same wallet or to a different wallet.
• On the amount view, click Send Max on the upper right corner. The wallet will now calculate how much bitcoin can be economically sent including miner fees. 
• Finish the send process by clicking Click to send on your desktop computer or sliding the Bitcoin B all the way to the right on your mobile device. Then confirm.

If you don’t mind waiting for your consolidation transaction to confirm, you can use a lower fee. How to Set the Miner Fee in the BitPay Wallet.

To view the amount of inputs your wallet has, on the home screen click the wallet icon on the tabs bar (bottom), under Crypto wallets and keys select the key which contains your wallet and click Addresses. If your wallet has a high number of inputs, then it will cost more in fees to consolidate these inputs.

The Send Max feature may not include all inputs if the wallet calculates that an input in question is not worth including in the consolidation.

For example, if there are many inputs that amount to 0.0001 BTC, it could increase the size of the transaction. If the transaction were large enough, the fees could increase to 0.0002 BTC. In this case, those inputs (funds) cost more to move than their value. 

Any inputs which are too expensive to consolidate remain in the wallet as dust. The only thing that can be done with small, currently outputs is wait for network fees to decrease before sending.