I have three suggestions, based on the releases for WPS so far:

First, and I am aware that this is being discussed, the service window should be monthly. Let it roll over on the first of every month. With the caps on allowed services that have been present since the release (excepting, of course, the dagger raffle at EG), even if someone manages to get 10 services each time the merchant is offered, you're looking at no more than 120 services in a 12-month period, assuming the service is offered monthly. Keep the service caps, as they are by far the better way to prevent these services from getting out of hand.

Second, services purchased through currencies that have a direct cash cost should never effect or be effected by the purchase of services using silvers. If someone gets 10 services for silvers in August, those 10 services should never change the cost for additional services purchased using Bloodscrip that same August, and the opposite should be true as well. I would put General Tickets and Seashells in this category, as well, as they cannot be obtained without someone spending money in addition to their subscription costs.

Third, and more of a general suggestion: unrestricted (scripts/other properties don't interfere) slips that can be redeemed to directly place X services of either a select WPS, or X padding, Y sighting or Z weighting, with the certificate being FLIPpable to change the service type. These can come in different sizes for different prize tiers. A jackpot-tier might offer 100 weighting, 125 sighting or 150 padding, and be unrestricted as far as how many can be redeemed on an item. A tier 5 might only offer 20, 25 or 30, and have a restriction for only 1-2 being redeemed on the same item. A tier 4 could offer 1 service, with a restriction of 5 or 10 per item.

You slam your mithril spikestar into the slowly descending meteor!
A large piece of the meteor shatters, exposing an opening!