Amazon has been notifying sellers they believe can help them with their Prime Day for the past few months. These deals are setup well in advance to allow a seller to ship inventory to FBA and be ready to go. Lightning Deal submissions for Prime Day are closed.
Regarding fees for lightning deals, these are not exactly fixed. Depending on the deal volume (i.e. unit counts) and the price of the unit and its reduced pricing, Amazon will determine the fee and allow a seller the option to approve or deny the deal.
You may find your account has lightning deal recommendations already. The [Lightning Deals Dashboard|https://sellercentral.amazon.com/merchandising/ref=ag_lghtdels_dnav_xx_T2#/dashboard/create] will show you these. Also, on this page, you will see the announcement mentioned earlier that Prime Day submissions are closed.
Regarding your question about sales volume – we have participated in last years and will again in this years prime day. Our sales, for items we were approved a deal for increased by close to 600%. However, our returns also spiked a week or so after when buyers decided it was not something they really wanted. We were happy though because those costs where off-set by the extra sales we had.
Even items we did not have deals on increased too. Simply by the increased buyer volume and searches, it is likely you will benefit in a bump in sales with or without a live deal.
Of our inventory not involved in deals, we saw close to a doubling of our sales for the day.