Depends on the contract really. Way back when, I sold a video (on VHS) - had to use a duplicating house to make the quantities I needed, and from retailer to retailer the payment plans were different - some payed net30/60/90 from receipt, some paid as they sold, some paid on receipt of the tapes. You'll have to see what you can negotiate in your contract.
Be careful on pay as they are sold - you have to make sure that 1) they are being honest on their accounting and 2) that they are paying in a timely manner.
In terms of a percentage - that also fluctuates from retailer to retailer - we sold some at a 40% discount, and some as low as 60% discount depending on quantity and contract. It will all depend on negotiation, what they can move, when they will pay, etc. Just be aware you will probably have to outlay some initial cash to get the ball rolling and you'll be in the red for a bit until they pay up. Just make sure you make enough - depending on the retailer, they may also expect you to eat the shipping (so, if the DVD would retail at say 19.95, and you gave a 50% discount you'll bring in 9.98 / dvd, subtract the duplicating cost ($1.75 using Kunaki as an example) then the cost of shipping (say between .30 and $1.50 / dvd depending on how shipped) so you will be making ~$6.73 / unit before any other expenses of yours)... You need to find out more about what this particular retailer tends to do with their wholesalers.
In terms of Kunaki - I don't know
One other suggestion - if they supply a contract they want you to sign, read it over carefully, and you may even need a lawyer to read it over to make sure you fully understand the terms the retailer is getting you to sign for.
