Ouch!!!
Last time I got spark plugs for my motorcycle through mail order, they came via the US Postal Service! I'm sure someone ships them that way to Guam. If not, you could always move to the mainland!
At least save that link to sparkplugs.com so you have a handy cross-reference chart to make it easier to compare prices and applications.
You might check Woody's Auto Supply (online), they ship worldwide via USPS Priority Mail FLAT RATE for $20 (still high) or $11 for the first pound to US Territories.
Here is the shipping rates for Woody's, and they DO carry those plugs:
US Territories
USPS Priority Mail Shipping
Weight (lb) Cost
1: $10.95
2: $12.95
3: $15.95
4: $18.95
I've never bought from them but hey, it's an idea.