I use Eurotunnel for both motorbike and motorhome crossings. I use my Tesco vouchers for this as a a £30 bike crossing only costs a £10 voucher.
If you phone Eurotunnel if you know you're going to be early or late, the staff are very helpful, for example last year when I came back a day early with the Vee, I called and they gave me a range of times and costs including no further cash to pay options, although I did have to wait an hour and a half to cross at no cost.
If you want to use your Tesco vouchers, look up your crossing price on Eurotunnel's website, surrender the right amount of vouchers to give you the rewards token equivelent at 3 X the voucher face value, for example my summer trip cost £23.50 of vouchers to get the £70 rewards tokens I needed.
Tesco post the vouchers to your home, then phone Eurotunnel on the number given on the bumpf with tokens, get your reference nuber, fill in a single page form , include the deals tokens in the envelope and post special delivery to eurotunnel, simples and job done.
After a day or three you'll get an email confirmation for your crossing. It is actually essier to do than it sounds, I haven't paid for a crossing in several years, making two bike and two mootorhome trips per year.