I might be repeating known info so bear with me--- but make sure I'm getting this right:
Factory warranty covers parts/labor for defects 4 year/50K. Does not include any preventative/annual services.
Service Plans are offered as 4 years (overlapping the factory warranty) and 4 years + 4 years (8 year/100k). These cover ONLY annual services and must be purchased around the time of delivery.
After the initial factory warranty for parts/labor expires, even though the owner may have purchased the extended service plan (4 + 4), new repairs/parts/labor would NOT be covered-- even if discovered during the remaining 4 annual services.
For parts/labor coverage for years 4-8 of ownership (up to 100k) there is an Extended Service Agreement which can be purchased at any time during the warranty period, though the price may change. Repairs have a $200 deductible and are subject to proof of prior service per Tesla's recommendations (see Service Plans above) in order for ANY repairs to be considered covered. (Read part C of the ESA).
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In summary: If you want "bumper to bumper" coverage for 8 years/100k, you must purchase BOTH the Service Plan AND the Extended Service Agreement, $3800 + $4000 = $7800.
If you EVER want to purchase the ESA later, you must have all annual and recommended services done within Tesla recommendations, in order for the ESA to be valid (so 4 services at $600 (price subject to change) or the 4 year Service Plan for $1900). Skipping services during the initial 4 year ownership period could render any later repairs under the ESA "invalid" (though I'm sure the website will happily let you spend the $4000 on the ESA without warning you).
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So, if you want to be a "proper owner" and mitigate expenses... you pretty much HAVE to buy at least the 4 year service plan since skipping annual services renders you ineligible (potentially) for coverage under the ESA, should you want it later. That being the case, why risk the prices rising-- lock in the service costs now rather than counting on them being $600 four years later (Roadster prices just went up to $750). If Tesla lowered annual service costs, you'd still come out ahead as long as the price was $475 or more.
In ownership year 4, you have the option of "repurchasing" your warranty at whatever the current price is ($4000 now). If you deem it worthwhile, you are also committing to paying 4 additional years' worth of annual services (to keep the ESA valid) if you have not already purchased the 4 + 4 Service Plan. On the other hand, if you decide the Extended Service Agreement is not worth the current cost, then you can now start skipping annual services as you see fit based on the history of your particular car.
What to do? $7800 seems like a lot for full warranty coverage... on a car that ostensibly won't need it.