You need to consider whether you truly need and will benefit from the SEO service they are offering to provide you and if that particular service is good value for money.
Sure, go with a reputable SEO service, but do know that you will often pay more for the privilege when you can usually get the exact same thing cheaper from a lesser well known SEO provider.
Like all things, research is your best friend, shopping around for the best deal, and finding an SEO provider that is right for your needs.
Sometimes the SEO they do is overkill and can end up hurting your rankings in the long run.
And you could end up paying through the nose for that.
Whatever you do, you will need to make sure you know exactly what you're getting and be able to verify all of it.
Sometimes you may even be able to pay half now upfront and then the rest again once the work is completed.
Sometimes you may be able to wangle a deal for reoccurring SEO that's paid for monthly or annually.
SEO is an ever ongoing thing and for tough, competitive niches, isn't hardly ever something you buy once and that's it.
SEO is constantly evolving and shifting with the times and is much more than just buying some links, slapping them on and that's it.
And SEO isn't always just something you can outsource to someone else.
Not only can this prove to be very expensive in the long run deeply cutting into your profit margin.
There are no guarantees that it will work and any SEO provider that does guarantee top rankings is lying plain and simple.
For that reason, you should learn SEO yourself and do your own SEO which isn't and doesn't need to be impossible but does guarantee the work, and it's cheaper.
Just some food for thought for you.