I am making the same assumption as sandyclaus: to wit, that you are referring to California courts (and law). To that extent, I agree in part, disagree in part and think one important consideration may be overlooked.
Costs that the prevailing party is entitled to recover are filing fees and costs of service. There is a division in that some judges will not allow the costs of personal service unless the plaintiff has first attempted the less expensive method of service by mail. Others will without first trying mailing. There are good reasons for each position and no statutory mandate for one or the other. The only caveat is that charges for personal service must be “reasonable in accordance with the practice of the industry”. – i.e. your brother-in-law can’t charge $500 for walking to the door.
I don’t know what sandyclaus means by “payment agreement”. Subject to clarification, it sounds like a formal agreement made at the time the basis for the future debt is established, such as a contract or promissory note that includes an interest provision. If that were the case, how do you categorize an invoice that provides for “interest of 1½% per month on any amount outstanding after 30 days”? The debtor need not have agreed to that when a purchase order was issued, and contract law does not allow that to be an implied provision of a contract or note.
Per CC §3287(a), every person with a right to recover damages that vest on a particular day, and are certain or capable of being made certain by calculation, is also entitled to recover interest on the damages from that day, except for periods in which the debtor is prevented by law or by the creditor from paying the debt. (That takes care of the invoice problem.) The statute allows recovery of prejudgment interest in causes of action other than contract; the crucial factor is whether the damages were readily ascertainable (see Marine Terminals Corp. v. Paceco Corp. (1983) 145 CA3d 991,193 CR 687).
Per CC §3287(b), Every person with rights to recover damages based on a contract action in which the claim was unliquidated may also recover interest on it from a date, fixed by the court in its discretion, before judgment but not earlier than the date the action was filed.
Per CC §3291, the plaintiff may claim interest on damages in a personal injury action on damages resulting from a tort. When the statutory conditions are met, the statute mandates prejudgment interest but only for the personal injury portion of a more general total recovery (e.g. the personal injury but not property damages resulting from an auto collision).
Finally, under CC §3288, in actions other than contract and in every case of oppression, fraud or malice, interest may be awarded “in the discretion of the jury”. The Small Claims judge, sitting as the trier of fact, has discretion to award interest under this provision.
I believe that the reason the vast majority of litigants ignore prejudgment interest, despite a possible entitlement, and the “Catch-22" for you is the fact that the filing form doesn’t exactly provide a space for interest, does it? It asks how much the plaintiff claims and asks how it was calculated. If you have evidence of a debt for $2,000 and you’re asking for $2,600, you must give notice to the court and the defendant that your claim includes interest for three years at the legal rate of 10%.
OP, you appear to want to ensure that your judgment is for your maximum legal entitlement. You ask about bad checks and interest on these. It’s not clear if all or any were made good but, if any weren’t, pre-filing compliance with CC §1719, would entitle you to treble damages on the face amount up to $1,500 for each, added to the claim. Maybe you've already done that, because it could be worth much more than consideration of interest on the checks. But, if you get statutory damages on the checks, you won't get interest on them. Conversely, each constitutes a contract, and you can choose interest instead.