Abrechnungsmodalitäten - die Kostenübernahme erfolgt auf Basis der gültigen Abrechnungsmodalitäten zwischen A und B für herkömmliche Garantie- und Kulanzfälle.

Both these sentences will probably require some paraphrasing on your part, but this is essentially what’s being said:
Terms - Costs will be reimbursed on the basis of the existing [current] [valid] arrangements [terms agreed to by A and B] for conventional warranties and accommodation warranties.

periodisch-summarische Abrechnung - die Abrechnung der betroffenen Schadensfälle erfolgt mittels einer periodisch-summarischen Abrechnung zweimal pro Jahr.
Periodic summary settlement - Losses are settled twice a year; settlement is carried out on a summary basis for the period


Abrechnung f
(com) accounting
– settlement of accounts
(com) statement of accounts
(com) invoicing
– billing
(Fin) clearing
(Bö) liquidation
– settlement
(Bö) contract note

Modalitäten fpl
(com, Fin) arrangements
– features
– terms
[reference for the above: Schäfer, Hyperbook]

Kulanz = obligingness [Zahn];
aus Kulanzgründen = in order to accommodate (oblige) someone [Zahn]
Kulanzregelung f
(com) broad-minded, liberal settlement of customer's complaint [Schäfer]
The basic idea of "Kulanz" is that the customer is right – even if the manufacturer feels that he isn’t, they will accommodate customer’s claim in order to keep customer happy and loyal.

übernehmen = to take over, to accept, to assume, to take on

I took some liberty in using the term “reimburse” above, but that’s essentially what the German says. The manufacturer will absorb the costs incurred due to losses (I assume), and they will do so on the basis of existing agreements that govern warranties and those cases where the manufacturer feels it would be wise to accommodate the customer.

I’m sure you’ll be able to come up with a more elegant turn of phrase – hope this helps, though!