guarantee:
"a formal assurance that certain conditions will be fulfilled, especially that a product will be of a specified quality" 
noun: 1 a formal assurance that certain conditions will be fulfilled, especially that a product will be of a specified quality.
'Guarantee' is to a standard of quality and is not time bound... and is non-extendable.

The difference is legal, not linguistic. Both terms are meaningful in the context of a contract or bargain. 
A guarantee is a promise that, if a thing is not of a certain standard or does not fulfil some condition, 
the original price or consideration paid for the contract or bargain will be returned

warranty:
"a written guarantee promising to repair or replace an article if necessary within a specified period." ...
 noun: (pl. warranties) 1 a written guarantee promising to repair or replace an article if necessary within a specified period.

Warranty has a time limit, e.g. for 12 months... and may be extendable to 24, 36 or 48 months.
warrantor may choose to repair the product, and not give a refund, is what distinguishes this from a guarantee
A warranty is a term of a contract, breach of which gives rise to a claim for damages, but not the repudiation of the whole contract.

Warranty is only relevant to the repairing of articles
Guarantee is generally given by manufacturers whereas the warranty is provided by most of the retail sellers or distributors


So, FSS offers a Warranty. NOT a guaranty.